Batman's 'one rule' is that he never kills his adversaries.' It is not always followed by Flash. Reverse Flash, his arch-nemesis, once attempted to murder his would-be wife, Fiona Webb. Barry became infuriated and accidentally snapped his long-time adversary's neck. Notably, Flash from the future has blown up Gorilla Grodd's skull, attempted to murder his earlier self, and even killed Wally West from the future.