Perhaps the events as depicted in Force Unleashed video game are simply Alliance propaganda.

This is how I've always used it.

Whilst I loved the story, I disliked how OTT Starkiller was, and how he was able to mow down legions of foes with almost no apparent effort (not to mention take down two Sith, one after the other). So, where I apply FU as canon, I tone down his powers and have his achievements very hard-won indeed: he didn't fight his way to Vader, but had to hide, fight where possible, and hide again; he didn't mop the floor with Vader, but scored a lucky hit that was dealt just before he himself was to be cut down; he didn't yank a Star Destroyer from the sky, but rather awe-struck onlookers and Rebel propaganda chalked the cause of an already-falling ship to the powers of their Force-using hero.

So, in my games, the end effect is the same and the story remains true, albeit with a more believable protagonist who had to suffer through much before the end.