Why might a crime be considered manslaughter rather than murder?

The difference between these two charges is the intent of the accused. For manslaughter, a prosecutor typically must only prove that the accused acted in a way that was reckless or negligent that resulted in the victim's death. For the other charge that cannot be typed here because apparently the system in its infinite wisdom accepts all kinds of adult, sexual and other questionable topics but thinks a formal legal term is "vulgar", a prosecutor must prove the accused killed the victim intentionally.