You’re kidding?

One way to describe leftist ideology is forced sharing. The "forced" part is what you're looking for, that violence comes in when people refuse to share (for example one particularly sore point in parts of Eastern Europe is that socialists hunted - literally - people who attempted to be self-sufficient in nature ... including during famines).

Authoritarianism comes in when people start making the obvious point to large groups that they would, individually, do better without a leftist state. Why does authoritarianism come in at that point? Because it's absolutely, 100%, true, almost by definition.