What exactly is the reasoning behind Satan ruling Hell?

It's Christian fanfic. There is surprisingly little about Hell in actual scripture, definitely not the "hell" we've come to expect from modern media. Most depictions of hell originate from Dante's Inferno, not from the bible.

Wow there's a lot of bad faith answers in here. Just because you're upset about Christianity doesn't mean giving a terrible uninformed answer is helpful.

As for the general answer to your question (no longer a Christian, but did missions work for many years), the predicate of Christianity is that God created everything in the world. Therefore, by basic extension, everything's place in that world is to worship its creator (and not itself), because worshipping God in this paradigm is its fundamental natural purpose. Not doing so is a perversion of that purpose and leads to dissonance in your soul, sin, etc.

Satan did it first, and as some commented, was the first to be in this realm of far from God by worshipping himself instead of his creator. Like anyone with a terrible addiction, you tend to want to get other people to support that addiction, so humanity is easy game (Adam and Eve). In this sense Satan is like the older sibling teaching bad habits to newer siblings.

Taking this further into the future, you now have Satan and humans who will be occupying this space of "God not being there". In that time, if none of you are God, it stands to reason that the strongest will prevail, i.e. the angel who got you to sin in the first place. The bible doesn't paint too much about him explicitly in regards to his role in hell. This idea of Satan just not being part of the harmony of putting god at the center explains the strange way that he and God still have a conversation in thr book of Job since hell is rejection of God from the self, not the other way around. What we can infer from other parts of the Bible about his role, like the numerous examples of human sacrifice and other degenerate activities for "false gods" the picture of Satan is that he hungers to use humanity to make himself a God and that he's a lot stronger than us were he to do it (hence the idea that he rules hell).

The last little bit on this is at the book of revelation. In the end, the prophecy is that Satan and humans are thrown into a lake of fire. There is no governorship here. This is the final judgement where all who are irredeemable are destroyed. The period of rule most Christians probably got the idea you asked about is from earlier in revelation when the righteous are raptured and the earth is run by Satan (again, not on purpose, but by consequence of being the strongest). Ultimately though, his fate ends up being destruction, not rule. So for a while, he has apparent power by nature of being more capable, cunning, and self-serving over others who have rejected God and therefore have none of the one things he cannot surmount.

You asked a good question, with a very interesting answer. A lot of very naive atheists seem to like to Google bible passages and then forget that there is such a thing as internal consistency. In a paradigm like Christianity, you need to start with it's fundamental postulates and walk from there. Not project your own completely different viewpoint onto it and act smug when they don't match. At the very least, Christianity is a centuries old story with some interesting internal consistencies and comments of human nature. There's a whole fascinating side discussion of this stuff on whether or not Satan is being given the same long grace period that humanity is to accept God.

Too cool for God.

the bible says satan causes illness but then god causes the firstborn to die, causes plagues, and so on.

the bible says god is good but god is a judgemental ass, who raped a woman and impregnated her, and who forces suffering onto people. while satan welcomes all who come to his realm, to better themselves.

satan's the real protagonist, not god. and the bible is so self contradictory that science alone holds the answers.

edit: likewise, if you look at greek mythology, Hades is actually pretty chill compared to the rest. He still abducts Persephone to make her his wife, but he treats her decently and lets her visit family and friends. Can't say that about Zeus, who gets angry randomly and fucks anything, regardless of whether it's moving

In the book of Job, Satan is depicted more like a friendly prosecutor or advisor to God than an enemy. He and Yahweh are like frat bros torturing the poor schmuck.

From my understanding hes just the first prisoner, not actually the leader of hell

Okay, bear with me for just a second. There's a cartoon called "Hazbin Hotel", which it told from the point of view of Satan (Lucifer's) daughter. In her version of their origin story, Lucifer gave humanity free will. But because humans are, well, all over the place, this allowed for evil to exist. Which lead to the creation of hell as a place to put these evil souls.

As Lucifer's punishment, he was sent to rule over hell, so that the only outcome he would ever see for giving humans free will was humans turning evil. Over & over again for all eternity.

Yes it's a cartoon. But I thought it was very elegant here.

I think its kind of a matter of interpretation? The "why" is that he either refused to bow before men (who he thought of as beneath himself), or, the less fun option, he just didn't follow daddy's wishes and got sent to the corner.

As for his "award", that too is kind of...opaque. One way to interpret hell is as the absence of creation, as in literally outside of Yhwy's world of creation. So either a new second world for unworthy creations to go when they end, or just literally a void of nothing, because to be removed from God's world should be to be removed from reality.

Ideas about hell being a place where the unrighteous are tortured largely come from later, non-canonical stories, with the biggest name easily being Dante's Inferno (which itself seemed to be largely about the author taking jabs at people from his own era that he didn't like or agree with iirc).

I'm not a scholar on the subject tho, so take it with a grain. This is just what I've heard, and I'm quite sure that I've heard some incorrect and half-true things before.