Good people can do evil deeds... At a basic level, what they did is wrong. I know as a player I would not feel confortable doing that (my real life values always overwrite what my character (or NPCs if I dm would do honestly).

They felt it was deserved, so it's some sort of vigilante justice, which is not justice, but a reskin revenge. I don't think what they did makes them evil, but they surely are angels now.

I dm often and I noticed whenever a really ahrd battle ends, players will often do extreme actions to vent out the stress and everything else. It's probably what happened here. If anything, it shows you played with the emotions well, so props to you!