Was it the fist bump Dennis did with Bill Skarsgard that caused him to shoot all the guards?
At first i just thought he finally snapped and couldn't take working there anymore but then i remembered the fist bump he did with Bill Skarsgard's character, and it was played off like something happened when he touched his fist with some ominous music playing.
I'm thinking whenever you come in contact with him something bad happens to you afterwards, the roommate he had came towards him and didn't he say "Don't touch me"? next thing you know he's being taken out dead in a body bag.