Trump says, regarding illegal immigrants, and specifically transnational gangs like MS-13, "You wouldn’t believe how bad these people are. These aren’t people, these are animals."
The danger in denying basic humanity to a certain group of people is twofold:
1) It's too easy to use that to justify TREATING them as non-human, and
2) It's too easy to start spreading that non-human category to include other people you don't like.