Yeah, I know, will piss them off, but you are the person being offended by their killing of innocent beings and having fun doing it.
Of course, you have to be willing to lose them as friends.
In reality, I have not chosen to piss off ALL my flesh eating friends, but I have sent emails to friends telling them to never send me photos of animals being killed or pictures of dead animals that have been killed. That I'm not the type of person that can get pleasure from killing other beings, nor do I enjoy seeing other beings killed. That unnecessary killing is Evil and I don't want to be part of Evil.