In the case of the Rice shooting it looks justified. A terrible tragedy, but justified. How would anyone know it wasn't a real gun? That kid was stupid.
In the Brown case I don't know what happened, because the eyewitness testimony is conflicting, and because the physical evidence can be made to fit any number of situations. What I will say is this: Whatever Officer Darren Wilson did, he acted according to his training. The system trains these guys to be killers — isn't it being a little disingenuous of us to punish individuals when they are?
The system is seriously fucked up. That's not Darren Wilson's fault. His police career is over regardless, so if there was any concern about him being out on the street, it is allayed. Justice, such as it is, has been served.
Funny how justice always ends with a white guy getting away with killing a black kid, though. And people are starting to notice, I think. That's what all those demonstrations in New York, Los Angeles, Boston, etc., were all about. This doesn't feel like justice. But it seems to have fizzled out. Maybe the Thanksgiving holiday interrupted it. Still, the interval between incident and outrage is shrinking. The next one will be upon us before we know it.
Yeah... they're not really trained to be "killers", per say; there's some 400 police homicides a year, which while tragic, means you're about as likely to be struck by lighting as you are to be killed by police (although you're an 1/8th as likely to be killed by lightning). They don't really kill people that often.
As for the evidence, it conflicts with the idea that he was shot in the back or that he didn't attack the cop at all, so we kind of know that's not the case, and that the conflicting witness testimony is more or less false; in addition, his finger prints were found on the gun, and among other things, the story of the officer was corroborated with multiple witnesses at the scene, more or less meaning that the story the kids told was at least stretched or altered, and the story the other people told matches up with the evidence and the officer's statements.