I'm from a family of hunters and so I'm not squeamish about hunting or guns even though I'm not personally interested in either. The hunters in my family are respectful of wildlife and concerned about animal suffering. I've found it goes either way with people, and the ones who get into because they are into proving how tough they are by taking it out on animals are not the ones I'd trust to have weapons around me, much less share a home or a life. Likewise it's concerning that he did this in front of his friend, like they are showing off to each other how macho they are.
We don't know enough about your husband to give any real advice about what he's like or why he did this, but if I were you, I would definitely be on the watch for other red flags and also dig around to see if this is a part of some bigger pattern. As for myself (and we all have our own feelings on stuff like this) I would not be capable of being in a relationship with someone who kills bunnies for fun. But I've also spent enough time around farmers and gardeners to know that bunnies, as cute as they are, can be very destructive and they reproduce at an alarming rate, and sometimes killing them is a form of pest control. It's upsetting to do it, but I lived in a rural area for most of my life and I had to kill gophers, possums, some snakes and a huge number of rats/mice. It never became pleasant work, but it was necessary, and people who think it's not usually just benefit from living in a place where that sort of killing was already done for them. Now I've luckily never had to kill a bunny myself, but they can be a pest. I'm saying this because perhaps there was a reason they had to kill the bunny and the jokes about it afterwards were just a way to act tough about it? Because if it really was just because "it's fun to kill things" then I'd be very alarmed.