...but I'll spit it out anyway.

The kid was most likely obsessed/possessed by his suicidal father.

Unfortunately, this violent suicidal scenario plays out countless times all over the world. Once a suicide consciousness signal is on the loose, those who have developed frequency relationships with the suicide must be doubly alert. This naturally includes all friends and especially family members. Suddenly folks who have everything to live for sink into unexplained depression and then worse.

Children often haven't the mental discernment to discard irrational thoughts and make pliable tools for these deranged mentalities with vendettas. All the child would need to do is think about his dad and feel sorry for him or sympathise with him and he would be setting up his receiver to that now discordant and distorted signal. Just like smoking, drinking, eating - violence obsessions are powerful and contagious. One look at Palestine or the Middle East will illustrate the generational continuity of extreme violence within families so afflicted.

His father would be likelier to have a motive to kill the grandfather and his girlfriend. While the child would be more likely to desire and lash out at all his school-mates who ridiculed, ignored him or whatever.

The murderous/suicidal obsession doesn't fear death because they are already dead and have little attachment to their victim's physical anatomy. When so possessed, the gun is all too easily turned at themselves.

Or to borrow Rob's train of thought - the suicide thought is a virus/worm and the kid's system has now been overwritten & corrupted.

That's my take on it. Naturally, I don't expect too many to share it.