Intervening in other people's fights is almost always a bad idea.
Dipping in when other people are arguing over money that has nothing to do with you is worse.
Doing so in a bar makes it much worse.
Doing so ending with a punch that kills someone makes it much worse.
The Detroit News: Police: Bar fight ends in man's death, criminal charges
Apparently Thing 1 Eric Hysell, approached Thing 2 Mark Thomas Foster claiming that Thing 2 owed him money.
Things apparently got heated quickly with some pushing most likely in a typical barroom Monkey Dance
So far so bad.
Then Thing 3 Andrew Werling intervened, apparently to break up the fight.
Breaking up a fight apparently has different meanings to different people:
In the tussle, Foster fell to the ground and Hysell was still standing when Werling punched Hysell in the head . . .
After being punched, Hysell hit his head on the patio door, or on the doorknob, and became unresponsive. He would never wake up again.
Yes, punches can be lethal, especially when the punchee hits a hard object on the way down, and subsequently dies.
Werling certainly didn't improve matters by intervening in a fight that was none of his business, and is now facing a manslaughter charge.
Physically interfering in other people's altercations rarely ends well and is just not a good idea in general.