You don't have to be a sociopath to lose your temper with a jerk like this one.  

By the time this video happened, the officer had put up with this guy's mouth, attempt to avoid arrest etc. for quite awhile.  Unfortunately, despite 14 years of being a police officer, he couldn't just ignore it.   Maybe because he had been putting up with that stuff for too many years; maybe because he had other things on his mind that night.  For whatever reason, he '"lost it".  

"It" being his professionalism as a police officer.

His attempt to cover up his lapse with that bogus "upper arm" entry in his official report now makes him useless as a witness in future criminal cases.  

The defense attorneys will now start their cross examination with "Isn't it true, Officer Sharpa that on September 11, 2016..... and finish it up with "Have you ever been convicted of a crime?"  and he will have to acknowledge Official Misconduct and Fourth Degree Assault convictions.  That's called "Impeachment".