Cyclist kills motorist

Probably not, no. But then it's not quite the same thing is it? That would have been entirely an accident, whereas punching someone is very deliberate. Though that's not to say a light admonishment would be appropriate.

I agree, punching someone is clearly very deliberate, whereas dooring someone is clearly a result of not paying attention to what you are doing.

If not paying attention to what you are doing results in someones death then that's clearly a bad thing, but in the (entirely theoretical) case of "death by dooring" I reckon the driver of the car would get off without any real sanction.

This is based on nothing but my prejudice, that said.