The truth - in the real world is that all ideologies have a violent fringe.
But the left is invested, intellectually and emotionally, in the idea that it is only the right that has a violent fringe, because right-wing ideologies - libertarianism, conservatism - etc. are inherently violent in a way the Left is supposedly not. They replay right-wing shooters over and over not - or not just - due to a deliberate plan to shout down their enemies, but because this confirms their world-view. Conversely, they downplay left-wing shooters because in their world-view these are just cases of 'lone fringe maniacs' that don't mean anything - which they don't.