@weeabootrash that is very much something called Consequences.
If you can foresee that leaving someone at your murder alive is going to get people killed, you get to choose murdering him, saving lives and potentially having society see you as someone who kills unarmed, helpless people and charge you of murder, or you can hand him over to the police and let their fate be decided by what society thinks is fair.
But then again, if literally anything can happen on a fantasy world or comic book, then even murder isn't necessarily going to help.
In comic book worlds, if you want to argue that murdering bad guys is right, because otherwise they would just break out of jail, I can argue that murdering them is just as pointless because oh look a magic user or godlike being brought them back to life.
It's funny that you jump to real world conclusions just like the girl here - assuming murder would stop a bad guy and not allow them to be revived through some dark magic means, or have their consciousness copied, or cloned, or be dragged out of hell, or any other sort of method.
The protagonist straight up does what you want, and stabs the spider guy to death.
Great job, murdering really was the final solution that you wanted, isn't it? He's dead, can't hurt a single person ever again and is much better than locking him up.
Oh wait, he's brought back to life because there are variables that don't exist in the real world. (And the spider can just possess him again, and you are back to square one.)