Ok, first of all, he didn't slam his head into the wall...he tackled the guy and he hit his head on the wall as a result from that. It's not like the officer grabbed the guy by his waist band and then spiked the top of his head through the wall.
Secondly, it looked like the guy was kiiiiind of trotting away from the police officer, which can be viewed as running from the police. I'm interested in seeing if there is any video footage that occured BEFORE this video clip started. From the looks of it, the man was trotting away from the officer, possibly trying to get away (who knows?). If that is the case, the officer had every right to take the guy down, especially since he was believed to be a suspect in an assault. If you are a suspect in an assault, the police aren't going to give you the benefit of the doubt and treat you as if you won't be a problem. They are going to take your ass down because you could be dangerous. In this case, the man wasn't the guilty party, but that officer was told by a witness that he was involved in the assault. So, the officer, taking full precaution, treated the man as being the suspect in the assault.
And, let's be honest here...
If this happened anywhere else, other than right next to a wall, this incident wouldn't even be an issue. It's unfortunate that this happened, but it was an accident. The guy's head hit the wall by accident; the officer didn't maliciously pound the guy's skull into the wall or anything, so why treat it as such? Oh wait, that's right...because he's a police officer. I almost forgot.