Clarified that for you above.
The question is, should he have been punished more severely for the previous violent act? I don't know the answer to that question, I don't know that situation.
To quote the movie "Law Abiding Citizen": "It isn't what you know, it's what you can prove in court."
If they didn't have enough admissible evidence, they could not have kept him locked up any longer than they did.
If you know you have a homicidal unstable racist living next door to you, and you don't take the necessary steps to provide for your own defense, you haven't got any right to complain that nobody else did enough to defend you and yours.