Big guy, schmig guy. I'm 51 years old and 5' 8"....and if I ever witnessed something like that, I'd be all over the sumbitch like a cheap suit.
He'd feel it, too. Trust me.
Remember...this is Chicago my former hometown.
Consider this senario. Possibly you know this is a cop. You get involved and hit the cop. Luckily there was a camera working and aimed at the incident. And, please don't think this is the first time a cop has done something like and had the whole thing washed away. But, what if you had hit this cop...and other Chicago cops don't like that regardless what another cop does...and the victim subsequently intimidated? Likely you would have been beaten severely by the other cops and thrown into jail on some trumped up charge. You would go through legal hell. No fun living in the city with a past record as a cop beater. On paper you attacked a cop and there may have been no camera or victim willing to back up your story.
Makes you think twice about getting involved...and I consider myself a brave person.