These are some theories and just a few things to throw out. Take it or leave it, and read at your own discretion. Some of them also don't go that well with each other, so it's not one big but several smaller theories. Let's start with the Tower. There's a story about how a man built the Tower so he could reach the star. But in practice it looks like it's the other way around. It wasn't the man who built the Tower, but the star/god who built the Tower, so that a man can climb it, but not without testing him and deeming him worthy, maybe to become a star/god himself.
So we kinda know that there were people already living in the Tower, by the time the 10 and Zahard came in. It is also known that Headon was the first one to meet them when they came into the Tower. Now it seems very likely that there were Irregulars before Zahard. If we think about Irregulars, SIU kinda stated that you need a special power to enter the Tower, or that the Tower itself kinda chose them. So where did they all go?
What if Guadians and Demons are fallen Irregulars of ancient times and while the Guardians deemed worthy of some kind of reward and got a position within the Tower the Demons might have failed to meet the standard for that or went crazy beforehand, but their souls/essence is still lingering around trying to reach the Top by any means necessary. If you think about a Tower without native people it seems very much like the first stage of Revolution. Maybe the initial climbers were meant to go through that kind of process by natural means, similar to Baam (I expect that he might have went through the very first phase or at least a part the first phase of Revolution in that cave), but in the long process of climbing some of them might have gone crazy. So in order to avoid that people were put into the Tower, to entertain the Irregulars, and the process of Revolution was attempted to be simulated by other means, like later on with the Hell Train.
Now let's think about Headon for a bit. He's the first Guadian Zahard and Friends met when they entered the Tower and the first they made a contract with. Now while Zahard climbed and conquered floors it was always mentioned that he's just conducting tests on behalf of the Guardians and that they remain the true masters of their floors, but I'm not sure if they actually stay in charge after their Floor Tests have been passed. My assumption is that Headon is actually gaining the highest position (below the actual God of the Tower) over the Floors Zahard conquered and that he's actually attempting to gain influence over all of the Floors of the Tower. That's why he wants Zahard or someone else aligned with him to keep climbing the Tower.
I actually think that Headon might have been the first Irregular inside of the Tower, back when there were no Guardians. He was tested by the God himself, but didn't pass and became the first Floor Guardian afterwards, wishing to some day get to the star/God (again?) to confront/challenge the God once more, maybe not personally. Now let's take the 3 other Guardians we've heard of to use some of the spare information that we have on Guardians. We have the 2nd Floor Guardian, who seems to be aligned under Headon as he let's him interfere greatly with his test (FUG people in charge of the Floor and Rankers playing around in the middle of the test). We also have the 135th Floor Guardian, that is said to be extremely hostile. Now the 2nd Floor Guardian didn't seem to mind to have his test passed by a bunch of people (Season 1 plot), but that wasn't a test where his supremacy over his Floor was on the line. What if the 135th Floor Guardian is so hostile, because he is the vanguard of the unconquered Floors and doesn't want to lose his power to Headon, at least not without the contestants of the tests to show that they surpass/ed him (It is quite possible that the Guardians got a severe power up, when they received their position, so it might be enough to show the Guardian greater potential than his former Irregular strength rather than having to actually be stronger than him in his Guardian form). He wants to prove his worth as someone who himself came that high and only wants to let those get past him, that show that they are truly worthy. There could also be a rivalry between Headon and that certain Guardian (assuming Headon was the first Guardian he could have relationships with all or most of them as they all had to get past him). I also find it possible that Headon had a hand in Zahard actually getting as far as he did. Maybe Headon is considered so strong and influential by the other Guardians, so that some of them didn't go too hard on Zahard and the 10 as to not upset Headon, but they still had to play their role to test them properly. Maybe the 135th Floor Guardian knows that everyone coming to see him to get his test actually had to have come through Headon, and is therefore so hostile or was very hostile towards Zahard and the other people who have seen it. Then at last we have the dead 43th Floor Guardian, and Headon seems to be interested in those events, maybe feeling responsible or just curious as to who was strong enough to do that, but not being part of his group of little adventurers.
Now let's come to the deal Zahard made with Headon. I think that's on of the main aspects in Tower of God, the way Zahard became King and why Luslec had a problem with that. I don't think it was something petty like revenge on Luslecs part, but instead something very important, a key aspect for all of the people of the Tower. So why was such a deal beneficial to Zahard and Headon?
Let's start with Headon. The Tower is meant to be climbed, but nobody has ever reached the Top, at least it doesn't look like that under the current god (maybe there were different gods at different times). So Headon does want someone to climb the Tower, but he does want that person to be in his pocket. He made a deal with Zahard right away and he also seems to get Baam with the deal he made with Rachel. Urek might not have shown the interest and maybe not the talent for Headon, but Headon is said to look for Enryu after he slew a Guardian.
So Headon doesn't have the best track record when it comes to having people reach the Top of the Tower under him, so I guess that Zahard and the 10 told him that they could do a better job at improving people while climbing the Tower. I guess Headon agreed to that under the condition that they have to clear the Guardian tests before putting their own system in place of each Floor, because obviously if their teacher isn't even able to pass the test how could he expect their students to get stronger than that, so that was the minimum requirement for enforcing their system. Also if you think about it, it seems interesting that Headon actually stayed in full charge on his Floor, obviously because the Irregulars are the people who are actually supposed to climb the Tower and are therefore very important to him.
So the deal made Zahard King and made it so people from the Tower are allowed to climb it, but in exchange Zahard has to make sure the tests are conducted properly according to certain standards. As I've mentioned before I think that originally the people of the Tower were just there to entertain the Irregulars, so Headon probably didn't feel too bad about making Zahard the King of them. Zahard and Headon then promised the people of the Tower that they could get anything they could ever imagine if they got to the Top, but in the end that is probably not something any Regular could ever get, as the Guardian tests are meant for Irregulars. In the end they are just used to maintain the testing and as stepping stones once an Irregular enters the Tower. That's what I think Luslec found out about Zahard, that he is simply using the people of the Tower, promising them things that aren't meant for them, it's the complete opposite of what Rachel told Baam in that cave. Rachel told Baam a story about the chosen people (Regulars) in the Tower, but it seems like they weren't chosen for that at all. It's the Irregulars (Baam) who are the only ones who have the right to actually climb to the Top of the Tower and no one else. People risk their lives, kill each other, betray one another just for power, ultimately just to stand over other people. I don't think Luslec has a problem with Zahard being King just because he came out of nowhere taking all for himself, because, at that time, he was the strongest in the Tower, who claimed that position.
Now there probably still lies responsibility in Zahard climbing the Tower further up towards Headon, so he probably has 2 choices here, the same as Headon, I suppose. He either tries to climb up further on his own (not alone; from Headon's point it's Zahard himself climbing) or he finds someone to do it in his name.
Then there's also the aspect of the workshop and human experiments. Trying to artificially create stronger beings, maybe for training purposes or to eventually use it on Irregulars themselves to make them stronger or to develop weapons that make them stronger.
The last thing I want to talk about is the Zahard's Princesses. What if their acual purpose is to bear children?
As mentioned before, Zahard's job is to make sure that the Tower is climbed further under his or Headon's name. So naturally there would also be the possibility of having an extremely powerful child, whose power you could use to gain great strength. Zahard's Princesses probably live thousands of years and are all beautiful. They also don't seem to have many responsibilities. It's like their entire life is a test, being beautiful, but never able to be close to anyone for all their life, which is pretty much forever, since they are pretty much immortal, almost set up to fail at some point. Maybe the children of the Princesses are used to make Ignition Weapons or specifically the 13 month series stronger or they use them in a sacrifice (like Hoaquin's Demon) to give strength to someone, maybe Zahard himself, while they seal the Princesses themselves. Then there's Heice Zahard, who refused a 13 month series weapon and the Eurasia Family to forbid their daughters to become Princesses, probably for a reason.
Then there's also the theory about Eurasia Enne Zahard being Baam's mother. Baam was thrown out of the Tower (or born outside) so that he could enter it as an Irregular and eventually climb the Tower further than anyone else in the Zahard Empire. Enne might have realized that she had been set up and fount out what they wanted to do with Baam and then went crazy. If that's the case then that's somehow interesting how Zahard could have Enne locked up somewhere, while Headon has Rachel in his claws.