Rio police officers arrested for killings

According to this article eleven officers in Rio have been arrested for the massacre of thirty people in poor areas of the city. This is only the latest example of Brazilian police killing poor people for little or no reason. The article mentions that the motive for the killings perhaps relates to the fact the the officers were mad because several other officers were arrested for being caught on video disposing of a body. There are so many things wrong about the situation that I don't know where to start. It's sad to think that poor people in Rio are as likely to be preyed on by police as protected.