From the comments section:
The shooter was also a Trump hater, so he was a Leftist, which explains to me a lot. Leftists are very emotional and can't handle stress, and tend to have knee-jerk reactions to everything. The guy couldn't handle the stress, so he lost control and exploded in violence, just like most Leftists (antifa anyone). The guy was already set to explode with Trump winning (all the Leftist rhetoric), and losing at this event just set him off. But yeah, they need to have armed security at these events, period. Just the presence of armed security might have dissuaded him.