You know, someone over on TTH was disgusted with me for this: they believed Robin's vengeance kick made him evil and he should totally be over it since Spike has a soul now and therefore isn't his mother's killer. Bzwuh? Grey areas, people.

One doesn't just get over trauma like that; everyone deals with it differently. Getting a totally fresh start might be the best thing for him. Much as I appreciated Spike as a character, and was really peeved with everyone else's behavior in the Lies episode (including Buffy), I had to admit he (meaning Robin) did have considerable reason for his attitude.