Another lemming pointed out a hypothesis that explains all this:

To some, good and evil are something you inherently are, not just something you do.

Barring some very, very specific circumstances (e.g. baptism), the die is cast. From that vantage point, it's easy to have shorthand for good/bad guys, regardless of what deeds they commit. This is where corrupt leadership and propaganda slot-in by inventing new and exciting ways to judge people.