To your character, what does revenge mean? Does it mean "an eye for an eye," or "repayment with interest," or something completely different? What is your character willing to forgive, and what would he or she never forgive?
Revenge isn't about justice. It's about destroying someone because they've hurt you. I have taken revenge in the past and it did not ease the pain which had been done to me. But I would do it again. Some things just can't be forgiven. Some people just don't deserve your patience or forbearance.
I would take revenge against someone who killed my teammates. I would take revenge against someone who tried to kill me (self defence withstanding). I would probably take attempts at mental torture or destruction of anyone in the team personally. Those who set out purely to cause pain deserve no mercy.
I don't know if I will ever need to take real revenge on someone again. That damn spirit from the cult in Salem is the most likely target right now. I don't hold it accountable for B's death, we splattered his killers, but if it comes after Ajax again it is going to be sorry. I need to talk to the doctor about a way to pin the damn thing in one place so I can kill it next time.