"It's just a game" -- a game that the other people were invested in (emotionally and time-wise: he said that they had already played 3 sessions). If you spent three multi-hour sessions doing something and then a new member you invited wrecks it, wouldn't you be unhappy too?
Taking it one step further, he said " the rest was clearly disgusted" and yet didn't think that maybe what he was doing would be a problem? He could have simply asked the group at this point.
Sure, the colleagues didn't react professionally, but this guy messed up big time and seemingly didn't even notice that he did. A very quick and simple solution likely would have been to simply apologise the next day. "Hey, I'm really sorry about last night, I thought I was roleplaying my character and got carried away and didn't realise it would cause heartache." likely would have went a long way to smooth things over.