Eliza, two cases come to mind when I think of this.

John Hinkley shot President Reagan and several others to impress Jodi Foster, the actress. He has spent time in a psychiatric institution, and now gets to visit his parents every month unaccompanied (but with a GPS ankle bracelet).

Mark David Chapman killed John Lennon. He planned carefully although his reasons seem insane. He is in isolation in a prison. Yoko Ono has responded to each of his parole hearings by opposing his release. He is in isolation primarily for his own safety.

The police seem to be carefully documenting this guy's planning, so I'd guess they're trying hard to argue that he wasn't insane enough to just be institutionalized. Cases involving the death penalty are horrendously expensive and would give this person a huge platform to act out on. My guess is they'll go the same route as with Chapman, offering him life in prison for passing on the death penalty.