I have done and do some strange things (to some) including shooting, which some people would find as wrong as everyone finds the idea of what these people want to do, this a seperate issue.
Getting kicks from just killing in the most bloodthirsty way possible is totally wrong, by anyones standards.
Do these friends play gory computer games? What will they move on to, killing their granny for a few quid, trying to kill their whole family so they can be adopted by a rich family? These are two stories out of todays newspaper, I bet the teenagers that committed those crimes will have started out hurting/killing animals for fun.
I agree with the majority of posts(all?), if you can't talk them out of it you don't want them as friends, and even if you don't take part but stay and listen to them planning and bragging about it later, without reporting it, you will be accepting their behaviour, so encouraging it to possibly move up a gear.