We don't know what motivated this terrorist. However, it is not a stretch to connect the dots between the Soros-financed resist movement, the soros-financed antifa thugs, the hysterical democrat attempts to stage a coup against Trump, the implicit approval by the media and leading political and academic figures of campus violence and attacks on conservative figures, etc etc and this kind of incident.
Violent anarchy and unrest are staples of the left, having been used most notably in the Russian revolution. Relatively small groups who are prepared to resort to violence can have surprising influence. See the Iranian revolution, the IRA terrorism, Cuba, Venezuela and a host of African countries, where armed men upset the status quo.
The CIA has fomented this sort of unrest repeatedly in countries around the globe whose leaders did not obey their orders. Now the Saudis and their former allies the Qataris are financing it. Get people out in the street, paralyze the country, then when the government tries to restore order, get the media to run hysterical stories about "killing their own people" etc.
We are not there yet but neither are we in the first inning of getting there.