Of course it's a matter of personal taste, but I found Andrew McCarthy's performance worked very well. Clay is a low key guy in the movie and Julian is mercurial. I think both actors struck the right note there. As you know the Clay character is totally different in the book. Bret Easton Ellis was not thrilled with the film's interpretation of his characters.
I think the ambiguity of Julian's final action is intended. We're meant to draw our own conclusions. From what Mr. Downey himself has said, it would seem that Julian wants to finish everything.
Bear in mind these are my own opinions. I've seen this film scores of times and this is how it always plays out for me.