I'm not saying that cops were in the right in every one of those situations. It just means there's no context in a majority of those incidents - which is critical to determine if the cop was justified in what he did.
Likewise, the context is critical for each of the examples you listed. . .
Mike Brown - So much evidence that contradicted the whole "hand up, don't shoot" narrative:
In summary, the DOJ report concluded: There are no witnesses who could testify credibly that Wilson shot Brown while Brown was clearly attempting to surrender. The accounts of the witnesses who have claimed that Brown raised his hands above his head to surrender and said “I don’t have a gun,” or “OK, OK, OK” are inconsistent with the physical evidence.”
http://news.stlpublicradio.org/post/why-did-justice-departme...
The whole incident took place after Brown had just committed a strong arm robbery and assaulted the owner of a shop. There's a mountain of evidence that proves officer Wilson was well within his rights to shoot Brown but I won't go down that rabbit hole at this point.
Eric Garner - I would agree the cops acted callously in arresting Garner. That should have been a standard arrest. Take him to the ground, cuff him and put in the car. The cops were wrong in this scenario, and should have done a better job of simply cuffing Garner and getting him in the car - especially when he kept saying he couldn't breathe. There is however, still some responsibility on Garner's part that he resisted arrest in the first place, which then resulted in the officers putting him in the choke hold that eventually killed him. Had he simply allowed the officers to arrest him without incident, he'd probably still be alive today.
Tamir Rice - This was an egregious act on the cops part. Not only did the callers mention that the gun was probably fake, the dispatcher didn't rely that to the cops who showed up. The cop also acted way too fast in drawing his gun and shooting Tamir immediately without hesitation or warning. A $6 million settlement probably confirms this was the case. Not to mention the officer in question had also been deemed "unsuitable and emotionally unstable" in his last position in a different county and shouldn't even been allowed to be a cop again.