> The difference is between manslaughter and homicide. We take age, sanity, and intent into account, but hitting someone with your car is illegal no matter your mental state.

Incorrect. Now, in most cases, hitting someone while driving will be at least criminally negligent and thus be some crime, but it's not impossible for hitting a pedestrian to be a non-crime because of the absence of the mental state required for any applicable crime.

> Likewise, tax fraud (intentional) is punished differently than tax negligence (unintentional).

"Negligence" is itself a mental state. That an act is illegal (though of different magnitude) when it is done through negligence as well as intentionally does not demonstrate that mental state is relevant only to degree of punishment, not legality. (There are strict liability offenses for which mental state is not relevant to legality, only possibly severity, but strict liability is exceptional in the law, particularly criminal law.)