Threat: What would you do if your son wants you dead over phone upgrade?

I wouldn't take the threat too seriously. He sounds like he's doing well and so I wouldn't expect such an act to be part of his behavior (he's not involved in drugs and drinking etc). I personally would dismiss the notes as just a phase. They like to get attention when they don't get things their way. What you have to do is be resolute and stick by your decision. If you change your mind then they learn they can get things they want by doing certain actions to make you change your mind. Like I said stick by your decision and make sure he understands your reasons for it along with you wont change your mind. You should talk to him as well about them maybe saying they are inappropriate. Personally I see nothing wrong with what you have done, however young kids don't always understand the big picture behind certain decisions adults make.