Very interesting thread. I'm currently working on an urban fantasy novel. I put an earlier draft up for critique at another site. In that version, the POV character has to shoot and kill a child. The boy was young, I forgot what age I said he was, 4 or 5 I think. One person said she couldn't possibly accept reading about a child being murdered and didn't know how any writer could overcome readers' abhorrence for child murder. Mind you, the scene showed the kid being shot, but that's all it was -- he's shot a couple times, I mention blood staining his shirt, he falls. That was the extent of the description. This reader didn't care what the reasons were for the murder, she just couldn't handle it happening.
Another critiquer had one of his own characters kill a kid, so he was fine with my story. But isn't it fascinating the different responses this issue will raise.
In the current version of my story, the POV character has a nightmare about committing the murder so it's shown in more condensed way, but the actual shooting is the same. No prolonged descriptions of where the bullets hit or brains flying, he gets shot twice and falls down. That's it. I'm hoping that scene won't lose me a bunch of readers, but I guess that's a possibility.
One thing a couple people did like was that the POV character woke from that nightmare in obvious distress over having had to shoot the kid. Since she's the protagonist these readers wanted to know that she was upset by what she had to do.
I'm leaving that in my current draft because the scene has a purpose but I'm a little nervous about it's reception! I feel better after reading this thread though, so thanks!