Because he doesn't like killing. He craves the danger but he doesn't like the idea of killing even the dog. That's why he closes his eyes. < also most people just close their eyes when then shoot a gun.
Well that would be an interesting story for the kids----- pinning for that caption.>>> the comment killed it! hahaha<<<just imagine John never reading it, or reading it and loving her even more (normally I ship Johnlock, but this is cute, too)
But John could. When John married Mary, he very effectively killed a part of Sherlock. Funny thing about this show, it really knows how to kill you on the inside.<< You can't kill Sherlock physically, but you can kill him mentally.