What would you call a person/character who's monstrous but only toward monsters?

There are people/characters who can justifiably be called monsters.

What would you call someone who has the same impulses as monsters but directs them at monsters only? Like people who were sexually abused as children and then hunt down pedophiles and torture them to death?

Or Dexter Morgan; Someone who got the "serial killer bug" but directs it only at serial killers not out of a sense of justice but because they don't want to direct their bloodthirst at innocent people whereas monsters are acceptable targets?