IMO, If Z's claim he feared for his life = Evidence that Z feared for his life, then there's no need for a trial, in fact for any trial anywhere where a stranger shoots another without eye witnesses.
Thankfully, the laws do not work this way or anyone can bring a gun to any fight and get away with murder.
Therefore the trial is to establish if it was *LIKELY* that Z should have feared for his life to justify shooting an unarmed pedestrian he tussled with.
If he shot without justification, then that's reckless killing/murder 2.
The likelihood of feeling fear for one's life varies according to the person, a 17 yr old girl, a child, an elderly, a disabled person would all feel fear at different degrees from Z.
The clearest evidence is of course serious injuries or bodily harm that indicate there is enough cause to fear for one's life. In the absence of such, psychological fear must be proven, and this fear MUST be reasonably established, eg. prolonged losing fight, isolated place with no possibility of 3rd party assistance or police help, opponent wielding a weapon, physical inferiority, mental weakness, etc.
There must be certain criteria to support the defense of "fear for one's life", otherwise, anyone can bring a gun to any fight and get away with murder.
IMHO.