why is the first instinct in certain people to make fun of people who are different?

We just did an interview with a guy at my oxford house (sober living), and while he did get accepted, we asked him to wait outside.

and everyone starting making fun of him for being autistic and talking about how 'weird' he was. Even my own brother.

the house president was like "its been said hes an autistic"

Me and one other dude were like, lets give him a chance man. He did get accepted, but man i hate that shit

Why are people like this?