The counter-culture fantasy
One reason for the popularity of a creature like Chris Dorner among the Left is that those who control the culture in modern America still love to think of themselves as the counter-culture. Nothing pumps up this fantasy like a heroic "rebel" myth, starring someone who actively went to war against The Man. Some of the less energetic liberals will create a modest buffer of separation, as in the case of Marc Lamont Hill: "Oh, of course I don't actually condone Dorner's murders, but I can sympathize with the people who do, and he was such an interesting man with such a thoughtful manifesto, and shouldn't we pay more attention to the issues he was concerned with..."
It's really not all that different from an earlier generation mooning over violent Sixties radicals, like Obama pal Bill Ayers. You wouldn't have to push the average liberal very hard to get them to admit that either (a) they've studiously avoided finding out anything about Bill Ayers' nefarious activities, because they can't process the idea that the Lightworker would be tight with a domestic terrorist, or (b) they think his domestic terrorism was either no big deal, or actually kinda cool.
Rush Limbaugh's been on a tear lately about how Obama worshipers don't hold him accountable for the results of his policies because they love dwelling in a fantasyland where huge, powerful, shadowy, and sinister forces are holding Obama back. They gobble up the speeches where he slams his class-war targets because they actually see the current President of the United States as a plucky underdog in the battle against those targets; his years of failure are "evidence" of the villains' power and tenacity. Dorner fandom is an outgrowth of that same deep-seated need for liberals to see themselves as rebels speaking Truth to Power; they're no closer to accepting that the are the Power, and have been for a long time.