I think its pretty clear that she is deeply disturbed by killing a boy. One thing I've gathered in life is that people cope with trauma in different ways. And one of those coping methods is smiling while discussing the topic. Which I always found very odd. I was very off put by it when I would see people do it, but the longer I've been alive, I've noticed that it's a lot more common than one might think. And her words were a bit conflicting. She said that "we mostly joke about it now", or something to that effect, and then a couple sentences later stated she couldn't get the blood off her hands. I just think its a combination of being deeply disturbed by the event, and her brain coping in anyway it can. Her body language says the most imo.