Who decides who's evil?

Beat me to it

Being evil and acting on the inclinations are two different things...if a person is currently acting out their evil tendencies, then I'd say yes it's justified if the actions merit it. If they just have evil thoughts but have not directly hurt anyone, then leave them be in this life for God to judge before the next.

No, in theory. But don't forget what Nietzdche said: “Beware that, when fighting monsters, you yourself do not become a monster... for when you gaze long into the abyss. The abyss gazes also into you.”

No if you killed a murderer

I think taking someone's life is seldom justifiable. What people might define as evil is subjective. Killing a killer

... would that make you any better than them?

only if they have killed or harmed innocents and are incorrigible

That's a question I won't answer. Don't ask me why... It's uh... Part of the uh... Yandere Code...*awkward and nervous chuckling*

What's a Yandere?(i actually don't know)

Uh... Well... I can't explain that though...

It depends if they are actually evil or not to others. If they are only evil to you and not to other people, then it would be considered a crime to others.