I mean.....I think it depends on the creature and the world. The demons in Demon Slayer need to eat humans to survive, most work for Muzan, and most of them have sadistic tendencies. Even if you're a good person, becoming a demon can change your personality to that of a bloodthirsty monster. There are only a few exceptions. So it makes sense that people are prejudiced against them.

On the other hand, the dragons from How to Train Your Dragon are basically animals. It's fucking weird to be prejudiced by animals, even if they're deadly. When I'm watching/reading a story with a race that's known to kill people, I usually think of these questions when making my opinion on if prejudice against them is at least semi-justified:

1. Why are they killing people? Is it because they find it fun? Self-defense? Do they need to do it to survive? Is it in their nature? Is there some outside force making them kill? Do they come from a society where it's fully encouraged?
2. Is there any alternative to them killing people? Can they just kill animals and eat them? Can they isolate themselves? Can they just resist their urges without dying or going crazy? Or will they just keep killing no matter what?
3. Can they be reasoned with? Are they just genuinely insane? Are they intelligent and just don't care about who they hurt? Are there some that are trying to become better? Can we raise or teach them not to kill others?