It is my opinion that all violent events must be judged within their context and on a case by case basis. The Cuban government absolutely should have stopped the Bay of Pigs, while Chun Doo-Hwan absolutely should not have butchered his people.
As for Authoritarian, the word is usually thrown against Socialists without backing up what it means. If I say I am in favor of a Socialist Republic, which extends Democratic control to Production and eliminates the idea of the Wealthy influencing the state, does that make me an Authoritarian? Yes, according to those abusing the word Tankie.
For your question, leftism is support for Socialism and movements towards it, while rightism is support for Capitalism and movements towards it.