I guess you've never been sucker punched. I have. It gives your attacker the advantage. (Not that mine needed one as a 10-year could take me down.) I was unable to get my bearings, defend myself or break free after the first punch. My SIL had the same thing happen to him, sucker punched in a bar. Unlike me, he is a large person and packs a powerful punch but he went down hard, breaking his ankle, game over.
If GZ had started the fight, i.e., thrown the first punch, then TM would have had some mark on him. And, FTR, if GZ had his "azz handed to him" then you are making the defense's case. GZ was physically attacked by TM and in fear for his life: self defense.
JMO, OMO, and MOO