He was 18 (adult), he just assaulted and robbed someone and there were witnesses. That kid was going to jail. He knew it. He decided he didn't want to go. He was huge (look at the video). He probably said to himself, fuck this, I can beat up/kill this cop and avoid jail. In for a penny, in for a pound. Do I know this for a fact? Not at all. Just a guess on my part.
Why are you so certain he would go to jail? In this case, "robbed" is actually petty theft and "assault" is pushing. He didn't run up into the store, pistol whip the clerk and then ran off with thousands of dollars from the cash register. In all likelihood, he would've gotten a slap on the wrist.