We need to be very careful to avoid using that baggage-infested word "terrorist" in this context until we know more. The fellow was a Muslim and he went berserk and shot fifty people dead. He had been investigated by the FBI and was not regarded as imminently dangerous. It has yet to become clear, in spite of ISIS claims, whether the attack was motivated by Islamic extremism (not a phrase I enjoy using, actually). ISIS encourages lone wolf attacks and would, naturally, want to claim this as their work. Even if he was not so motivated, yes, terrorism might be a word to use, but let's not forget the baggage that comes with it.