A couple passengers have said that they saw the wound on his head when they boarded the plane. One male passenger said that Slater was standing at the door to greet the passenger as they entered. The male passenger commented on the wound and Slater said something like: it's been a bad day. Maybe the cut happened on a prior flight?
What I understand is why none of the other attendants step in during all this (apparently). One of the others should have told him to clean up and put a bandaid on before greeting the passengers. Can't help but wonder if one of them did and Slater snapped at them. I can't think of a good reason for a flight attendant to do their job with an unbandaged cut on his face.
Bottom line, if he couldn't handle dealing with passengers he should have talked to the other attendants and asked them to run interference (since he couldn't go home sick).
I wonder if he was the head attendants on this flight. That would explain the other attendants apparently not interceding during all this. If he wasn't the head attendant, then whoever was should have interceded.