What should your punishment be for killing, in a freak accident?

Let’s say someone had just adopted a new dog. He was returning from the vet, after getting her shots, and while she was sticking her head out the window, she rolled the window up on her throat and started choking.

He made an attempt to free her, and wasn’t paying enough attention to his surroundings. He ended up coming up on the sidewalk, and hitting someone. They died a few days later, from a stroke during surgery related to the injuries received when he hit them.

What would be the most likely legal outcome, in this scenario?

He did not run, he immediately pulled over, called 9–1-1, and later gave his statement. The officer taking his statement said this was just a freak accident, and unfortunately they do happen from time to time.

He has never had a ticket in his life, let alone been pulled over. He was not under the influence of anything. He surrendered the dog for adoption afterwards, as the guilt of knowing she was unrestrained, or the possibility of it happening again was too much.