Actually there's interesting research on this.. I wish I could remember where I read it - I think a Malcolm Gladwell book.

In general, we only need to see someone being smart once and then we will forever label them as smart. It's as if being smart is an achievement and they've proven they're capable.

However, in general we only need to see someone being evil once, and we will forever label them as evil. It's as if that's their true self, and all the other instances of them being good are a ruse.