OK, here's why they can charge that. Homicide is a two part offense that requires a mens rea (mental state) and actus reus (action). If someone kills someone else, its homicide, because of the killing, the actus reus. Punishment is dependent upon the mental state at the time of the offense. Deliberate, planned homicide is generally first degree murder, because there was full out intent to kill, and some serious thought put into it. Second degree murder is lower because it doesnt require planning, only intentionally or knowingly killing someone or causing serious bodily injury that leads to death. This is what Z has been charged with. Then you have manslaughter/murder 3rd degree, which is generally a "sudden passions" crime (so overcome with rage that you killed, but the killing wasn't entirely intentional, and then criminally negligent homicide, which is a nice way of saying a really, really fucked up accident.
Here's the trick. With homicide, a higher charge includes all the lesser offenses. Thus, Murder 2 includes manslaughter, and Criminally negligent homicide, since they are just steps down the mens rea ladder. As prosecutor, if i can prove that sudden passions led to the homicide, but no real intent, then I've proven manslaughter, not murder 2. The point is that a homicide still happened. All we are debating is culpable mental state, and that allows for convictions for lesser forms of homicide than the one charged. That's how lesser included offenses work. Assault and rape are not homicide, so you cant charge murder, since its not the same class of crime.