Beyond a reasonable doubt applies just as much to manslaughter as to murder.
Read your own post re self-defence, regardless of who started the fight:
"Alternatively, the person who provokes the initial attack may nonetheless claim self-defense if: (1) in good faith, he or she withdrew from physical contact, (2) clearly indicated to the other person that he or she desired to withdraw and terminate the use of force, and (3) despite the communication and withdrawal, the other person continued or resumed the use of force."
(1) Z was pinned down by M but tried to wriggle away.
(2) Z did not hit M and screamed for help while being hit.
(3) M continued to beat M for 30-40 seconds.
Plain as day. He acted in self-defence.
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