the unlawful use of violence and intimidation, especially against civilians, in the pursuit of political aims.
What is that guy (and the rest of these leftist, fascist fuckheads) doing?
They are unlawfully using violence against civilians in the pursuit of political aims.
They are, by definition, terrorists.
I'm not disagreeing with you. In addition to being a terrorist (assuming he was acting on anti-Trump motivations as you say), he's also an asshole (IMO) and a criminal.
I've said before on this forum that, on balance across all issues, I tend to lean left. But I don't understand or defend actions like this guy's, or the Coulter nonsense at Berkeley, or many of the other examples of lefties losing their grip in recent months. It's nutty.