I get why he did it, but it wasn't his decision to make. It wasn't his revenge to take. You can't just step in and make decisions for other people based on what you think is best for them. That's disrespectful and selfish and completely annoying.
I sound like a broken record, don't I? This is my main complaint about Angel, too, and about a lot of other Jossverse characters if I ever get around to ranting about them. Acting on this whole hypocritical I-know-what's-best-for-you attitude is one thing about fictional men that just really gets on my main nerve. It's obviously all over the Buffyverse. But not just that. On my other show, GH, the main men are gangsters who do the exact same thing. It really makes me mad!
Do I think Fred had the right to kill the professor? Legally, no. But do I think that if any one of Angel's crew had the right to do it, it was Fred? Absolutely. Also, the law isn't prepared to deal with mystical kidnapping and hell dimensions. That's why our heroes exist. In some ways, Fred is perfectly within her jurisdiction to take out the professor. The normal law won't, so our champions have to. In this case, Fred felt that she had to do so in order to move on. I was so mad at Gunn when he took that away from her. I looked at my husband and said, "He stole her vengeance! If anybody ever kidnaps me into a hell dimension, I get first dibs at killing him." My husband said, "Umm... I can't promise that." Ugh. Stupid men - even the non-fictional ones!
Rant over. Sorry.