For those who have read the book...

It's been years since I read it, but I seem to remember a lot of the characters being less sympathetic than they are in the movie. Also the plot is pretty different in parts. It seems like the general story line of the book was followed for most of the movie but then the entire last third of the book was cut out (or at least seriously compressed) and replaced with a shorter climax. Of course that could be the result of the movie being cut down from its original six hour run time. Anyway I haven't watched the movie in its entirety in a while either, so my memory may be inaccurate.

I never took Welsh to be a sociopath. Just someone who's heavily jaded prick with a lot of very harsh philosophical positions.
He seemed much more mean spirited than Warden, but not a sociopath. He even gives Fife and out of combat after he gets wounded by mentioning that there was a clerks position open in reach echelon when Band is about to assign Fife to one of the forward squads. It may not have been out of kindness, but it certainly was not unkind. Also his attempt to try to "save" Tella, again it was not out of kindness or caring, but it was something that had to be done to keep everyone sane. There were a couple other moments where he didn't seem quite so awful as well... just don't remember them clearly.

I have yet to see the movie, but if he comes across as a father figure that's rather off. He's clearly a bastard, and he knows it.