Hmm. I'll probly get flamed for this, but:
I have mixed feelings about this. On the one side I abhor any kind of cruelty to animals or people, it's unnecessary. But I like to try and understand why people do what they do.
You have to bear in mind our racial heritage as hunter gatherers. We are still physically and mentally at the same evolutionary stage as we were 300 years ago or a thousand years ago, when animals were food and that is all. Men in particular have cruelty built in, or how else would we have survived as a species this long. Yes, we are reaching a stage in society when we can transcend these basic urges, but you shouldn't condemn someone for displaying a misguided instinct. Yes, he needs re-educating, but he isn't sub-human, he isn't a monster, and maybe living in this society where everything is provided for us hasn't given him - or any of us- a chance to have an experience that will help him realise that he can transcend this instinct. I've never hunted. I've never killed an animal. I don't know if I would feel enjoyment (the built in survival response) if I did, but I'd wager I'd probably feel quite guilty. I'm not at all religious - I'm a militant agnostic, but being such makes me live my life with an almost Buddhist respect for all life. Y'know, just in case there is some kind of reckoning after we pop off.
This might be his chance to learn something, and rather than rejecting him outright and treating him with disgust, this is a perfect opportunity to teach him the difference between the instinct that drove him to it, and the way we're supposed to be now. He may, treated with understanding and conversation, come to realise his error, which he clearly did as his backpedalling seems to display. Or maybe he'll remain the same and just learn intellectually rather than instinctively tat cruelty is an unworthy behaviour for a modern man.
The other aspect of this is would you have reacted with such disgust if he had (and Im being metaphorical here) swerved to hit an alligator, or some wasps, or the AIDS virus. They are all animals. Rabbits as a species have survived by being domesticated, and domestication leads to fluffiness, cuteness and all the other visual and behavioural things which make us 'like' animals (see the Russian experiment with the wild and domesticated foxes - www.liveleak.com/view?i=b30_1372049732 ). The disgust is quite arbitrary, and based on a principle we can only hold to because we live in a land of plenty. The same conversation in a third world country may have gone something like this:
'I swerved to hit a rabbit but I missed.'
'That's a shame, we could have eaten well tonight.'
It is arbitrary, he's not a monster, and a protective approach towards cute animals is just a luxury allowed us by the plenty that we live in. SO don't send him to Coventry or whatever. Just talk to him. Tell him what you think.
I don't think you're being unreasonable to not like what he did, but I think you are being unreasonable to pigeonhole him as a cruel man and punish him instead of educating him and trying to bring him round to your way of thinking. You said yourself, he's a nice guy. As such he deserves your understanding, not your condemnation.