My understanding (not a lawyer) is the difference is intent to kill. Murder requires that the accused clearly intended to kill the victim, while manslaughter only requires that the accused intended to do the thing which resulted in the victim's death.
"Hard to argue", sure, but that's not the standard of a legal trial. The prosecution would need to prove beyond doubt that the driver meant for Rapp to die, not just that he meant to hit her with his car.
Also if you overheat Teflon it releases toxic fumes. You probably won't get it hot enough by cooking, but if you accidentally left an empty pan on a lit stove it could get hot enough.