Although you may be getting that bias from whatever you're reading, MOST mainstream liberals and progressives do NOT support the extremist left, including nihilits and anarchists, or condone violence by either extreme.
Back in the cold war days, the old Soviet Union was a big supporter and promoter of radical leftist groups. The leopard didn't change it's spots. Russia intelligence is currently supporting and manipulating the extreme right (easy to pick out with Russian based social media bot activity), and almost certainly the extreme left as well. Supporting the extreme left adds fuel to the fire - and actually strengthens the right extremists by making them seem like "victims."