He doomed himself.
You locked him up and threw away the key. That is your action, directly affecting his psychology, directly harming him. You may be the judge, the legislator, the juror, the jailer, the voter. You have to account for it.
You justify locking him up by protecting others, but how do you justify the harm you’re inflicting?
Then, you’re assuming agency on his part. Choice. The kid is 15 FFS, go back in your own life, consider how much, at that age, it was yours, or that of the environment. You also need to argue that he was the reason he killed, and not his environment. Humans don’t generally kill other humans, they also don’t grow up to do so, something must’ve happened to him and I very much doubt it was his fault.
Not to mention ARM chips which by and large were/are more efficient on the same node than x86 because of their design: ARM chip designers have been doing that efficiency thing since forever, owing to the mobile platform, while desktop designers only got into the game quite late. There’s also some wibbles like ARM insn decoding being inherently simpler but big picture that’s negligible.
Intel just really, really has a talent for not seeing the writing on the wall while AMD made a habit out of it out of sheer necessity to even survive. Bulldozer nearly killed them (and the idea itself wasn’t even bad, it just didn’t work out) while Intel is tanking hit after hit after hit.