Negligent homicide would seem more appropriate in this case.
I agree but even though his act led to a homicide, he had no prior record so perhaps the DA went easy on him.
In Washington negligent homicide is actually manslaughter 2 - he pleaded to manslaughter 1, a more serious charge. He was not just negligent, he was reckless. The level of risk is the same for both, but recklessness means he was aware of the danger of his actions.