Someone mentioned that they only defense this guy would have was insanity. I believe that he can;'t even claim this. This was all premeditated.
1. He ordered guns, ammo, and chemicals, months in advance
2. he booby trapped his apartment
3. he set his music to loop, trying to lure someone to open that door.
4. He purchased a movie ticket,
5. he went out the door, knowing full well what he was gonna do
6. he surrendered to police
Someone who is mentally ill would not pre plan all that he did. Nor would they have everything set up perfectly. If someone snapped, or had a psychotic break they don't take time to think about their actions, they just react. He took a lot of time to plan this out, so an insanity defense may not even work here. The only thing he may be able to claim is being a sociopath, and even then that won't get him off, he will still be doing hard time.
I think the only thing that might save him from the Death penatly is that he told the police that his apartment was booby trapped. This helped to save many more lives.