They really can't, if they want to keep believing they are moral people that always do the right thing.
If they accept that someone else might not be willing to tolerate the evil they tolerate, then by contrast they also would need to accept they are not as good as they thought they were.
I'm definitely not explaining it well, but this is called being a Moral rebel.
And of course, it doesn't help that the liberal mentality is very paternalistic, they have to constantly feel like they're the only adult in the room.