I've always thought that cases of child on child murder can't always be put down to the transgressor being a psychopath. Small children just don't have the capacity to grasp the magnitude of the consequences of their actions. Killing another small child would be not much different to them than pulling the wings off a fly.
The concerning thing is that I can somewhat understand a six year old smothering their infant brother. I know I am not a psychopath because I have been assessed, and the possibility discounted, however, like @10HailMarys I also tend to be completely emotionally detached, and although I'm perfectly capable of empathy, it's strictly conditional, and I find it impossible to muster any for things I don't consider deserving of it.
I have a much younger sibling that I can recall contemplating throttling on a number of occasions. The only reason I didn't is because I knew I'd likely be caught, and I didn't want to face the consequences. I had absolutely no time for them then, I still do not, and the only feeling they have ever invoked in me is 'irritant'. I can't recall any feeling of concern at my sibling no longer being around, on the contrary, I found that prospect quite attractive and desirable.
I have absolutely no time or empathy for any small children. The only thing I feel when I hear a baby crying is complete revulsion, and an urge to move as far away from them as I can, as quickly as I can. I have no urge to harm children, but I simultaneously have no urge to placate or soothe them either.
I do not feel this way with adults, in fact, I tend to be overly empathetic and care a bit too much. The emotional detachment helps me in my job, but I also think a total inability to empathise would make it impossible for me to do my job. I've never had any complaints about how I interact with people, nor have I ever had someone say they did not wish to work with me again even though it's made abundantly clear to them that is an option if they aren't comfortable with me. On the contrary, I am constantly given positive feedback, and management returns glowing reports.
Perhaps that's all just smoke and mirrors, but I don't think so. I'm just a bit perturbed as to why I can only feel empathy for adults and not children, but maybe that comes back to having a poor relationship with my own siblings, and only ever resenting them. I have no idea.