>Where do demons even come from? Hell?
Demons came from Heaven. They were originally created as angels, but they rebelled against God, and He cast them down out of Heaven into Hell.
>I thought hell is forever.
Hell is forever, or at least it will be. Certain demons can exit Hell temporarily and visit the Earth. Some are trapped in Hell, and have been since their fall, but others can roam more freely. On Judgment Day, God will seal all of the demons in Hell forever. Human souls typically cannot leave Hell until God resurrects their bodies on Judgement Day, after which the damned will simply be cast back into Hell to burn body and soul.
>If I can just become a demon to fuck with people then why should I worry?
You can't become a demon. You're a human, not an angel. If you die and go to Hell, there's no coming back for you until the resurrection of the dead, then it's just right back to Hell. Ghosts don't exist. Angels and demons can give living humans visions or produce appiritions of souls in Hell, but the souls themselves cannot leave Hell until the general resurrection.
>I mean, they would need some sort of mental capacities to make choices on how to tempt and corrupt people the best.
Demons are vastly more intelligent and knowledgeable than human beings and can reason instantly, but all of their talents and capacities are utterly squandered on evil and viciousness and rage.
>And trolling on the internet has caused at least one suicide. So, why should I worry when I could become a demonic shitposter with mental faculties, immortality, impunity aside from getting banned, etc?
You'll never become a demon. You'd become the plaything and food of demons in Hell if you went there. Hell isn't pleasant for demons or humans. It's like a prison with no guards where the prisoners can do whatever they want to each other but get no enjoyment out of doing so. Damned human souls have no ability to influence the world, and those demons that do have that ability will lose it on Judgement Day.
>Sure, satan would be my master or something, but what does he even do? Nothing.
Satan devours souls like a rabid lion. There isn't anything he wouldn't do to the damned.