From a filmmaking standpoint, it is tough to go the other way and make the main character utterly unlikable though. If you do that, you are not gonna make much money off of the film. A good example is Henry: Portrait of a Serial Killer. I would imagine that is one of the more realistic portrayals of how a genuine, real life serial killer would behave, and as a result, he's got little to no redeeming qualities and most people are revolted by the character in the film (or at least, I would hope that they are!). But as a result, the film would not have been funded by a major production studio as mainstream audiences won't go anywhere near it. I haven't seen anyone clamoring for a custom figure of Henry for good reason, though many want Travis Bickle from Taxi Driver, who had some kind of code and was portrayed in a much more likable way by Deniro, but was still a deluded psychopathic killer (like Rorschach in some ways). As long as your homicidal psychopath has a moral code that folks can agree with on some level (i.e., Punisher, Rorschach, Dexter, Bickle) folks get into it, and of course, folks also get behind over the top crazy/monster killers that couldn't exist in the real world (i.e., Jason Voorhees, Michael Myers, Patrick Bateman) despite them not being ethical in any way. I like some characters in both categories myself, but they are, in general, fundamentally different from real world killers.
