You’re probably right. I watched a couple more episodes, and now it seems more believable. I could see how the cruelty and fear would just make people nuts. It is emotionally disturbing to watch episodes where a Marine deliberately tortures an enemy soldier, though. I just watched the part where I think Sledge is going to cut the teeth out of a Japanese soldier, but another soldier tells him not to even though he has apparently already done so himself. That was one moment where at least some shred of the good made an appearance.
The book gives even more horrific detail on Japanese atrocities and how it affected the Marines fighting them. The book is also a lot better at explaining the what and why of how the Marines acted and reacted but that is because it is a book and can give a lot more detail than a movie can.