Nothing good ever comes of it.
Horror movies seem to fall into two categories, or at least the two types I watch. I don’t watch slasher films like Jason, Saw or House of Wax. Not my thing. But I will watch things involving cryptids, ghosts and the undead. (To a degree, zombies are out.) There is either the “travesties of science” types, the original Werewolf of London, the invisible man, Frankenstein. These are the “man has interfered with nature through science and no good will come of it.”
The other type is where man tries to thwart the will of God or the gods and feels that Death just isn’t good enough for some reason. Usually, these are religious types trying to punish blasphemers by denying them an afterlife and punishing them for all eternity while they are at it. They pridefully think they know more than God or their deity. They’ve missed this step in logic. They curse something to be undead for all eternity, whether it is a mummy or a vampire or a werewolf. (I don’t think the wolfman went into why they were cursed only that they were.) Suddenly, they have a much bigger problem on their hands than if they’d just killed the guy and be done with it.
Because then the monster returns for revenge or creates hundreds of others like him and suddenly they need to figure out how to kill this undead god like being they created and train special hunters to do so! They’ve created a worse problem than the blasphemer ever did! What hubris!
They would have saved lots of time if they’d gone the Greek and Roman route. You can’t thwart Death. Eventually, Hades will get you and his sense of Justice is legendary, though his humor is often misunderstood. Nothing good ever comes from trying to get around it. So, while creatures such as vampires and mummies and werewolves do eventually have a way out, it could be worse. They could be like Jack of Jack’O’Lantern fame, being locked out of both heaven and hell and wandering in darkness forever with only his lantern.