Stanley explains that he knew he would be a liability to The Losers, and that his inability to overcome his fears would put them all in danger. ... Remember, Stanley killed himself because he thought he was too afraid of Pennywise to do what had to be done.

What is Stan's fear in IT?

Fears: Like most of the Losers, Stan fears the adults around him, who have either ignored him, overlooked him, or misunderstood him to such an extent that he only finds comfort in the presence of the other Losers. But Stan's inherent skepticism often prevents him from fully committing to the battle against IT...

How did Stanley die in the bathtub?

Though the members of the Losers Club, as adults, can't remember exactly why they promised to return to Derry, they feel nauseous and afraid hearing Mike's voice asking them to come back. When Stanley gets the call from Mike, however, he immediately goes upstairs to the bathroom and dies by suicide.