For awhile there I doubted that Mitchell could be killed at all. But I'm guessing that since his powers were more mental than anything else, he was still a human being, and could still be killed by conventional means. Getting close enough to do it would be the only problem.
Although I'm liking the idea of the Q stepping in. Perhaps they were monitoring the situation and decided to 'allow' Mitchell to be killed, because if he escaped (which he surely would), he could threaten even them.