Hi, Harmon2, I think that most impulse killers qualify to at least a small coterie of friends as "nice guys". I think that in this case, some people saw both sides: At his friendly neighborhood baseball games, Clark apparently had to be physically restrained from attacking an umpire.
In rereading the personal accounts of Clark, I notice that virtually all the "he's a great guy" comments date back to high school and many were voiced by people who hadn't seen him since then. It's quite possible that the transition from being a popular guy at high school to being a rodent keeper rubbed this young man the wrong way. It certainly wouldn't raise most people's spirits.
P.S. Your argument gains force with this description by the ex-girlfriend which indicates that he seemed perfect at first, then turned into a controlling, apparently violent person. (It's interesting that the police apparently cautioned her not to speak to details related to what seems to be physical violence, although she did mention the forced sex:
http://abcnews.go.com/GMA/accused-annie-le-killer-raymond-clark-history-anger/story?id=8648090