So first, Syd Barret was removed from the band in 68 after a mental breakdown. It's suspected that he developed schizophrenia.

Second is the feud between Waters and Gilmour. They clashed constantly on the direction of the band, leading to Waters finally leaving the group. Waters left the group and Gilmour and Mason wanted to continue. This started an all out war over Pink Floyd continuing to exist. Waters finally resigned in the 80s, stating that fighting them would have ruined him financially. However, it's pretty clear that he continues to hold a grudge about how it all went down and what he feels is being banned from anything Pink Floyd related. Every attempt to resolve things since has resulted in them clashing. Neither party has been able to let things go, and it seems that Waters can't help but be antagonistic in general. This is a pretty short, but decent article about the whole thing. The whole history of Pink Floyd is an amazing read. What happened to Syd is just heartbreaking.

Waters has since gone and stuck his head into the Ukranian war, denying Russia committed war crimes because only the west has reported them, and that he's on a Ukrainian hit list. Waters has self reported that he dealt with a lot of anxiety, depression, and feelings of isolation. Apparently he went through 20 years of therapy. Still doesn't seem to be able to let things go.