If it attacks me, I kill it. If it's standing where I need to go and I know it will attack me if I get close, I prefer to be the one to deliver the first strike. In short, I have no qualms with killing bighorners if they present an obstacle to me. I usually leave the young ones be though as they don't attack on sight, and I often ignore bighorners if I don't have to get close enough to attack, at least I started doing so once I realized that they only attacked if you got too close, despite showing up as enemies on your compass. Not like I need the experience when I already feel that leveling is too quick even when I don't go picking fights I don't need to. The meat is nice I suppose, but since I'm not playing in hardcoe mode, I only need it for healing, and I've never found myself feeling that there isn't enough healing items to satisfy my needs.