I agree with WA and Kana. Not only can Fred bear (and has she borne) a lot more than it would first appear. As her boyfriend, Gunn should have respected her enough to let her make her own decision and to suffer the consequences. By doing the deed himself, Gunn undermined the relationship. Furthermore, as Kana already indicated, his actions did not produce the intended result. Fred was still complicit in the Professor's murder. She had still intended to do it. She had still acted on and announced that intention. He had still died. She still felt the burden of murder and fell from the pedestal on which Gunn had placed her.
I think Gunn acted impulsively. I can't say that his intentions were wrong, exactly, even if they were rather annoyingly macho and disrespectful of Fred's right to make her own decisions, but I also think that putting a little more thought into the matter could have saved their relationship and strengthened it in the manner that WA indicated. This is a very complicated moral area. Fred's revenge seems justified, but not "right." Gunn's actions seem justified, but again, not quite "right." I don't think there is any "right" when it comes to killing a human, even one who needs to be killed. I do, however, think Fred's way was less right than Gunn's way. If that man should have been killed by anyone, it was the woman whom he had quite literally sent to Hell and back.
