I remember seeing something where he never really felt like he was part of the band. The rest of the guys were hard on him because they never got over the death of Cliff. Too bad, dude's a good bassist.
They definitely have a lot of skeletons in their closet and I don't think we'll ever know the TRUE reason why Newsted left or how he felt. Lars spins it to make Newsted the dick head, Newsted spins it to make Lars the asshole (well, there's no spinning there but you get my idea), I know in the VH1 documentary Hammett took Cliff's death really fucking hard and both him and James truly never got over it.
IMO, of course.