evil is as evil does

im not sure if this is the right place to post this but id like to know how everyone feels on this matter. is it evil to do evil to stop evil?

example- a man kills your freind and threatens your family, cops have no evidence agaisnt him so he walks free, is it evil to kill him to stop him from killing your family?

or even! to kill a man you know commited murder but got away for it, and not a hunch but if you absolutly know.

iv heard alot of contraversy on this and was wondering what everyone thought.