the player they killed was running all offense and using DR to post big gains without being subject to retal, which has been a pretty big problem in Express and is basically against the rules (see sticky). So all players have a motive to kill him. Warster said the player would be deleted if he had joined GDI, but since he didn't, anyone could kill him or do whatever to him, so he was therefore permitted to play however he wanted. So people took him up on that and killed him.

but really, people join to kill people a lot on Express and don't get deleted.