Genocidal doesn't necessarily mean he's committed genocide; it means that he's prone to (and to some extent, fascinated by) mass acts of killing. The tweets about promising fire and fury on an entire nation of people (North Korea), telling Fox that we need to kill the terrorist families as well, etc. He gets off feeling like he's the most powerful man in the world.
He's a strongman, whose solutions often involve labeling entire categories of people as bad/evil (Muslims, Mexicans, the media). If you haven't noticed this you're either not paying attention or willfully ignorant.