A parent is legal guardian of a child until they are of age, or longer/shorter as prescribed by law (for mentally disabled, for people whose parents are in jail and ask for early adulthood, etc etc). Legally, they are responsible for the well-being and adherence to laws, of the child, and as such are granted specific legal duties and permissions. You're not kidnapping someone for grounding them, as it's a disciplinary measure. You're not allowed to break both your kids' arms because they got home late, because it's excessive force.
Well, yes; a police officer can take someone into custody, because he's allowed to by law. There's no need for someone of any of the three powers to jump in, because at that moment, he's following the law as laid down by the legislative branch and interpreted by the executive branch. That police officer is acting within the law, but he's very explicitly not saying someone is guilty, and he cannot be sentenced purely by him to anything. A short detention to ensure safety/prevent escape/etc. In judiciary systems such as Belgium or France, any other incarceration (yes, awaiting trial, etc) are decided upon only by the judge. in the US the system is somewhat different, but it's still someone else, specifically apointed for the job, looking at the case, who decides whether or not to keep someone in jail.
I agree with what you're saying, but strictly speaking, the Law can't proclaim someone judge over someone else - and certainly not judge and jury, without possibility of a higher appeal (with the exception of very specific instances such as spies and deserters in wartime).
It appeared to me (and apparently KO as well) that you meant that a police officer going rogue/using excessive force/killing someone without need/etc is, in certain cases, acceptable, as a deterrent. With which I don't agree, since any deterrence it would offer would be....what? "If you make me mad, I'll break my own rules and hurt you, but as long as you're not too bad, we'll stick to our rules"? That's ridiculous. You need to be able to deter them while staying inside the lines of the law.
Of course, in a best-case scenario, all of what I said is not applicable to this situation. I'm saying that I think it's a serious problem, under the hypothesis that a cop deliberately killed this guy.
