That's not what they did. They brought him to the morgue.
You know, where I grew up, it's very simple: if your actions directly result in the death of another human, you go to jail by default. "I only wanted to help" is a weak excuse, and "I only wanted to help by injecting a drug" gets you a longer sentence. As it should be.
I agree. I'm not saying that the police who did this were right or that they shouldn't be punished. I'm saying that with better training the kid might not have died. The idea that police could be trained to kill fewer people doesn't mean that they're currently not at fault when they do kill people.
Note that the police didn't inject the drugs. The paramedics did that. The police just held him down for that part.