Sounds like some jurors believed the guy was insane
From the story, this man may have been nuts and had a mental illness background but he was quite sane enough to plan and execute his crime, and attempt to avoid being seen, and also avoid its consequences to himself. If he could have, he’d have killed everyone in that building.
Personally, I believe ALL murderers are insant (well, except for the professionals who decided to make that their livelihood). So does that mean no murderer can ever get life in prison or the death penalty?