Kreese could have easily ended up in prison if they involved the police

Seriously, that would have solved everything. In the opening scene alone, they could get Kreese in prison for years. He assaulted Johnny, and full out choked him, like air completely cut off for a pretty long time. If Johnny is under 18, and I think he is, that's child engagement. Which can be a misdemeanor or a felony, and the felony can be a 2 to 6 year prison sentence. And with what Kreese did, I think he'd be guaranteed to get the felony, so he's going away for 2 years minimum.

Then, he assaults two other minors, Bobby and Tommy. Not serious assaults, but still assaults. With his assaults on Johnny, I dont think he'd get any leniency from a prosecutor for hitting Tommy and Bobby. Im not sure what he'd be charged with, since they're kids, but if it was assault, thats two misdemeanors and can be up to 6 months in jail. He would probably get the max for both boys.

And he attempted to assault Miyagi. Another 6 months. So if he got the max for each charge, he's looking at 7.5 years in prison/jail. And again, since he was so violent, I dont think get much mercy. At the very least, he's spending 2 or 3 years in prison.

And to top it off, as a registered black belt, his hands are technically deadly weapons. If he was charged with assault with a deadly weapon, he could get up to 4 years for each of the assault charges.

The bottom line, Kreese would be fucked if you got the police involved.