I don't know that he was going to shoot him no matter what, but in this particular situation the probability was near 100% and here's why.
Every cop in this country is being trained via simulation. Perp may or may not have a weapon, perp in simulator turns. Decide rookie. Too slow rookie, bang you're dead. This repeats over and over again hundreds of times in training. Too slow, bang you're dead, bang, you're dead, bang, you're dead. It's ingrained in the thought process at some point. Now let's add in the multiple real world situations where a routine stop turns into violent altercations and cops being shot. This reinforces that thought, too slow, bang you're dead.
Put it all together and the probability of getting shot if you're making the slightest wrong move in a high risk situation is just about guaranteed. Middle of the night, shots already fired, gun is spotted, chase is on, kid whips around quickly and what is the cop thinking in the .8 seconds he has to decide? Too slow rookie, bang you're dead.