I think the leftists champion such thugs because they know that for a communist "revolution" to take over, they will need to recruit a lot of thugs who think nothing about killing people. Most leftist professors hardly know one end of a gun from another, which is kind of a problem if you want to round people up at gunpoint and send them to the nearest collective farm. So you recruit stupid thugs who are satisfied with a little money and a little attention. This type of person is an ideal enforcer and is not likely to become a political threat. Problem solved!
That is a good theory, that's why they (clinton, carter) always pardon their thugs as a way to get an "I owe you one" from the bad guys.