The same is true of the right of course. Not everybody there approves of violence, but some do. Some have a long history of violence and harassment, and the president seems to support it.

Part of that, particularly from Trump and his supporters, and probably from Antifa and similar groups, is excessive tribalism. They hate the other side and are eager to hurt them. Trump supporters are certainly quite open about that. Antifa might actually be different. It's also possible they actually believe that violence is necessary in order to stop fascism. But any support they may get from the left is probably pure tribalism again.

Of course there's also another aspect to violence from the right: things like lynchings in the 1950s, harassment of women, violence against Muslims and other minorities. Those stem from pure bigotry and a belief in (usually) white male superiority and a right to punish others if they seem too uppity or seem like a threat. I assume most people on the right abhor this, and yet some of these still happen.