There's an inherent wish, at least for me, to like, or associate myself with the main, or one of the characters of whatever I'm watching, and usually that's the intention most screenplays are written with. But it doesn't have to always be so. It is, however, very hard for a film to leave a positive impression if you didn't like the character or story. I think that's why some people thought this movie wasn't good.
Personally I didn't agree with everything the 'Killer' said, but most of it was true, and the film itself gave away so many 'secrets', so to say. I suppose they're all, and more, known to any special detective authorities dealing with such 'cases', but it's definitely the first time I'm seeing something that accurate and meticulous shown in a major motion picture. Not to say it's not without fault - I'm not going to pick apart every little thing I've noticed, but the initial blunt that sets the whole string of events in motion could've been avoided. Such an experienced 'professional' wouldn't make that mistake, but then 'the movie wouldn't happen', so... let's assume he was sleep-deprived, and got too anxious, wanted to get it over with.
Pretty much everything after that was great, almost perfect, including the reasons, intentions, and the ending.