The premeditation began when he bought the gun. We know where he bought the gun...but not when he bought the gun.
You can carry a weapon on base if you register it.
So if he got leave, he wouldn't have shot anyone??
Sorry....the plan was to kill.
Witnesses said that he asked for a leave form and when he was told to come back later, that's when he started shooting.
He didn't enter a random building and just open fire.