The video is based on the theory that Snape was in love with Lilly and turned against Voldemort because Voldemort had promised to spare Lilly but then killed her.
Actually, this theory has some basis in the text so far. In Harry's repeated flashback to Lilly's death at Voldemort's hands, Voldemort is shouting at her to move aside, and she refuses. Why would Voldemort even try to get her to move? Certainly not compasion. Nor was Voldemort even aware that killing Lilly would trigger an "anceint magic" more potent than his own powers. It is plausible to believe that Voldemort had initially intended to spare Lilly (for Snape), but then decided to break his promise when it became the only way to kill Harry.
The genius of this video is how well the words match with the images. You need to listen with sound on, so you may not want to play it at work.