It is entirely possible that M was defending himself from a percieved threat by Z.
The end of the fight doesn't prove how it started. The pross didn't make their case.
They might have had they gone for manslaughter from the gate.
As it is, there wasn't evidence for murder 2 and going for it precluded all "accidental" killing verdicts.
"The law is reason, free from passion."
And that the conversation with Rachel ended at the very beginning of the confrontation.
No one will EVER know what really went down unless Z writes an "If I Did It." book.