The truth is that neither you or I know why he did so exactly, but it stands to reason that he cared about mass government surveillance in general and hoped that his leaking would prevent that through public outrage; that makes more sense than that he only cared about PRISM.
> The only problem is that the hysteria caused by people uncritically believing Snowden without looking at his documents helped lead to Trump taking over every branch of the US government.
Nonsense. Trump's demagoguery and subsequent rise had nothing to do with Snowden.
Given the dystopian path that the USA and the world are on with regard to mass government surveillance, I would say that the real problem was the lack of hysteria after his leaks. You also seem to be downplaying matters here.