Assuming he was an extreme leftist without ISIS ties, that would be another event of open warfare against conservatives from leftists. The attempted assassination of several Republican HOR members at Beseball practice seems to be an act of war from the left, well researched beyond the shooter's ability to carry it out.
We're in an internal war for control of the US. "Republicans" in the HOR and US Senate are enabling it to go on, even though it's clear they were elected to stop it.