In Canada (not sure how different it is in US or UK) homicide is broken into:
First degree = premeditated (planned) killing, hired killing (if you paid someone to kill someone else), if you kill a police officer (not sure if that part is right), or if you kill person while you are sexually assaulting someone, hijacking a plane, kidnapping, taking a hostage, during any sort of terrorist activity.. and a couple other special circumstances.
Second degree = all the rest, usually spur of the moment/unplanned. For example you come home to find your partner cheating on you and you shoot them.
Manslaughter = I might be a bit off but I think this is when you don't intend to kill somebody but your actions demonstrate recklessness/disregard for life/safety of others. I think if you kill someone while you are driving drunk this is what you can be charged with.. not sure 100% on that.
& then infanticide is separate as well.
But if MJ's doctor prescribed medications in combinations that could be harmful/lethal and he died while taking them, that would be manslaughter.