@kissane I think there are a couple of things...
Firstly, purity tests are an aspect of fascism, they're all about the individual subsuming and sacrificing themselves to an ideology or belief (usually that of the believer so it's making the other person subordinate and using the ideology as the lever to do so). It's not a coincidence that people who are judging others on these purist standards also see themselves as martyrs (and greatly underappreciated for their greatness). None of this is reasonable nor does it leave room for people to be human and imperfect. (We're all human and imperfect, even the people who believe themselves above all this.)
@kissane Secondly, there are also people who have a very hard time moving beyond binary good/bad thinking for reasons of nature/nurture and how their brain works. There's an aspect of splitting and projecting all bad onto others involved in this. But everyone can learn to move beyond binary thinking and it gets easier with practice (we can all use some practice doing this, our brains ARE wired for quick good/bad categorizations and quick reactions).