It matters because it spoiled the story.
In the beginning, we had the kind of story you find only in the movies: a perfectly innocent man who is unnecessarily killed by a bad cop. And then came the video of a large man brazenly robbing a store and manhandling a clerk who was half his size, if that. Anyone who has ever been bullied will likely have reacted to that image with seething memories from the past.
Did Michael Brown deserve to die? No. Even those who advocate the death penalty will agree with that. But the video cannot help but change the attitude people have toward him. To say it makes him a less sympathetic figure would be an understatement. If Michael Brown had only been arrested, is there any doubt that the video of the robbery would have engendered feelings of hostility toward him? Just as the robbery does not undo the fact that he did not deserve to die, so too does his death not undo the animosity we feel when witnessing the robbery.
People do not like innocent victims anyway. They interfere with the desire to believe we live in a just world. And so there is a natural tendency for people to blame the victim, as when they blame the woman who is raped for acting imprudently or even shamefully. They are perfectly willing to sacrifice her to Karma.
And so when someone is beaten or killed by a cop, regardless of whether he is black or white, the urge to blame the victim makes many people think to themselves, “Well, he must have been doing something wrong, or that wouldn’t have happened.” The video of the robbery merely confirmed what had previously only been imagined. There are those who cherished the image of Michael Brown as an innocent victim, and they are sorry to have that image sullied. But there are many who wanted the story spoiled anyway, and the video more than answered to that desire.
The video may not matter in a court of law, as in the trial of the officer who shot Brown, where it probably will not even be admitted as evidence. And it may not matter to a wise and merciful Providence. But it matters nevertheless.