"Except in any normal circumstance, take for example Zimmerman, the shooter would be in custody and at least have an examination or be charged with something before we go to selecting a prosecutor and preparing statements for a grand jury. None of this has happened and the police department is using this time to slander the character of Michael Brown."
A grand jury is what charges people in jurisdictions with grand juries, so you're just wrong there.
This is not really comparable to the Zimmerman case since here the shooter is a police officer rather than a civilian, so let's set that to the side. Since he's a police officer, I really doubt that they are arrested after firing their weapon in what they claim is self-defense. I could be wrong, of course, but I would imagine there are departmental procedures which the officer goes through and THEN he may be arrested if the evidence plainly supports that happening. But again, that could be wrong.
As for the slander, I don't see how releasing the video of Brown robbing the store is slander. For one, it's true. He did rob the store. Slander has to be untrue.
Perhaps the prudent thing to do was not to release the video, but it's certainly relevant information. The self-defense theory seems to be that Brown thought the cop was going to arrest him for the robbery, and attacked the cop to try and get away. Seems like a bit of stretch to me, but not unreasonable. But I agree, releasing the video to the public strikes me, on first impression, as something that probably should not have been done. But now that it's out, it's out. We can't ignore it.