What alt-left? Are you referring to community defense groups trying to protect their communities from Nazis? Dangerous to whom?
Antifa is a good example with their huge riots and firebombing of cars during Trump's inauguration.
The son of Israel's PM considers them a bigger threat to the jewish people than actual neo-nazis (surprising to me). http://www.timesofisrael.com/netanyahu-junior-says-leftists-...