Keep in mind that self-defense claim or not, the state has to prove 2nd degree murder, a predicate of which is intent. The witnesses could be critical and Zimmermann's injuries are going to be hard to overcome. I still say that the way that the two of them became proximate to each other is the critical factor. If the state can show that Z went after the victim, then the chance for conviction increases. If not, well ...
By the way, what would normally be the lesser charges included in jury instructions?
Voluntary manslaughter would be a likely charge, on which the jury would be instructed. I don't know whether the evidence would support a basis for involuntary homicide. The grading and elements of homicide charges vary slightly among the states.
But you are right that the foremost hurdle is that the prosecution must prove the elements of their case.
