Yes, absolutely. Purging a traitor cannot be considered evil, anymore than civil soviety can be considered evil for imprisoning or destroying a criminal.

As for murder, are we speaking of extrajudicial killing outside of a strictly military context? Then I would say no, that is the place of society to do what is right, and bring the transgressor to justice. To say that you are the only one who can define or bring justice is the most ridiculous sort of pride, and I've seen the unfortunate results.