Please, since you said your son has threatened/become violent over the computer before, get all guns and other weapons out of the house before this, (and KEEP THEM OUT!) Lock up "sharps" (knives, scissors, yard tools, etc)
Horrible to think of, but many parents/grandparents/guardians have been injured and even killed by children over computer usage.
Also, violent verbal threats, damage to property, and physical threats of violence/actual violence cannot be tolerated AT ALL! At 15, he doesn't have long before he's a legal adult, and he's already at the age where he can be tried as an adult in most states. If he does any of these things, PRESS CHARGES! He's less likely to be charged as an adult, and it may be an entry into care and treatement for him. Also, doing so may save not only his life, but yours' as well.