If you could have stopped and you didn't, you're an asshole and deserve whatever you get. You're right, the dog shouldn't have been on the road, but you're the (allegedly) higher-thinking creature in the situation, and you were able to make a choice. You chose wrong.
It was funnier, because the guy had a Menchi avatar. If you're a weeaboo, you'll recognize that as the name of the dog from that anime, Excel Saga, who encounters hilarious abuse at every turn.