Approx. 20.000 juveniles were arrested for murder or attempted murder in 2020 (per the US Dept of Justice) Shall we put them all down, as you say? Just to be safe?
Calling your position utilitarianism is interesting, in what way is really, actually murdering a child (never mind the massive amount of legal, ethical and emotional complications that entail) to prevent a hypothetical future murder maximizing utility for anyone?