Its because Frieren brings it up a lot and talks about it directly to their face. The evil things aren't just there, it actually acknowledges they are irredeemably evil and regularly mocks people for saying "they are just misunderstood" or "they can be civilized" with brutal horrific deaths.

The Lefties can feel how directed it is at their beliefs and reject it for that reason, whereas most "evil creatures" are often vague enough for plausible deniability.