I'm from Brazil too. So far, it seems that the policeman action were justified because the robber pointed the gun at the officer, he did the right thing in this case. The opinions were divided more by the fact the the media has shown this and it started a celebration that, in my opinion, is very poor taste. Like other said, I don't see nothing to be happy about it, it's just sad... even more considering that Brazil is not exactly the land of opportunity. I was robbed just like this and lost my car some months ago and I still don't see no reason at all for celebration, I don't feel "revenged".
Concerning the police, I don't see how they can be considered powerless here, since they are pretty much the official criminal gang of the State, doing whatever they please. Some months ago, a construction worker disappeared... recently it was found out that the police had suspicions on his involvement with drug traffic, so they took him inside a UPP (Pacifier Police Unit, it's a big vehicle), tortured him, everybody heard the screamings, then they killed him, took out the street lights to cover them and removed the body. That's how the "pacification" works in the favela.
A friend of mine knew a guy who enlisted for ROTA, a special police division. His "batism" was going to a poor neighborhood and beat the crap out of a random passerby, without motive. He was forced to do it, then left the corporatin in disgust. Recently, there were revisions in how the policeman has to report cases of "resistance to surrender" because ROTA was killing whoever they wanted and then forging reports saying that the victims resisted.
I can't agree with a police who works like judge, juri and executioner. The police operates like that since the 60s and this never resolved anything.