Here are the bullet points against Z as far as I see it.
(Keep in mind that I still don't know if Z was justified. We just don't know all of the facts yet. WildCat, who I have been batting this back and forth with, has said the same about M. We just don't know the all of the facts yet. If this all played out in a way most favorable to Z, then surely Z has been punished enough already.)
*: Z was keeping an eye on and following M on suspicion only. It is not in dispute that Z did not see M violate any laws.
*: Z was given a very serious sounding suggestion to stop following M by a police dispatcher. This suggestion was given during Z's pursuit of M and was within several minutes of the fatal shot being fired. Z responded to this suggestion with the word "Okay".
*: Several years ago Z was arrested soon after, and directly relating to, an incident where Z was arguing with multiple police officers and where Z pushed (or shoved) one of the officers. One of two charges was "Resisting Officer with Violence Battery on Law Enforcement Officer". The second charge, possibly the first charge reduced, was "Resisting Officer without Violence". Neither the argument with officers nor the pushing of an officer were disputed by either side in the resulting case and Z ended up having to "complete a pretrial-diversion program" to avoid conviction.
ETA: To try and humbly summarize this entire case:
This entire case hinges on who initially provoked whom into fighting. We do not currently have enough evidence yet to determine that.