system
Given the following post, respond as if you're commenting on Reddit:
human
[General] Why is it often said that if a hero kills a villain, they're every bit as horrible as the villains themselves?

I'm fairly certain most would say that there is an obvious and massive difference between villains who massacre countless innocent people for kicks or in pursuit of selfish gain, vs heroes who kill villains to keep such mass losses of life from happening.
gpt
Generally, there's four reasons. Descide for your self if any have merit.

One- some people do think killing is always wrong, no matter what. You can disagree, but some people do think that.

Two- often, this comes up when the villain is, at least temporarily, defeated or immobilized. Hence, it's no longer killing a threat- it's revenge. Is there a difference between killing an active shooter and killing the same shooter an hour later in prison? Many say so.

Three- under what authority are you killing? Most heroes have no oversight, no accountability, no backup and no mandate. Can they really be justified in deciding for themselves that this person must be killed? What if they make a mistake, ans they're bound to do eventually. Better to never do what can't be taken back.

Four- the slippery slope. You have the killer of millions, who sure, shank him. You have the pick-pocket, who don't. But what about the various shades inbetween? The bank robber willing to kill, but not going out of their way to? The person who killed their abuser? The ceo who ruins countless lives non-violently? There's a line somewhere. Some might think its better to draw it too high then not at all.

You can say all of these are wrong, but these are the arguments.