...the fact that this was blatantly fucking murder? Use your common sense.
In what world is pressing the barrel of a gun against the chest of a 4 year old, never mind pulling the fucking trigger, supposed to be about teaching gun safety? How is that anything other than premeditated murder?
Given the history of abuse in that household, I don't buy the idiocy angle. The other child watching knew what was happening and turned away so as to not watch it, for god's sake.
There was clearly an intent to maim or kill, perhaps to intimidate the other child. If it wasn't a gun in use, it would either be another weapon or a bare-fisted beat down.