I imagine you might find some data about this at the FBI. They collect this kind of data from the police all over the country. You’d find out how many murderers there are and how many people each had killed. As to the military, I’m not sure if the data are available. I’m sure you could find out how many people are estimated killed by American soldiers, but I don’t know if they would tell you how many soldiers had kills.
There are probably similar agencies in other countries that could give you these data. I’m not sure how far afield you want to go. If you are interested in other countries, you could see if the UN has data, and I’ll bet Interpol keeps track of some of this.
It is, of course, impossible to get any reliable data about this. In many war torn areas, no one is tracking anything. People are murdered without anyone else remembering.
I’m afraid that the answer to your question is something you may have to research yourself. It shouldn’t be too hard if you go to the places I suggest, but I’m not going to do this for you. It takes legwork and time I don’t have. You have to supply that yourself, unless there’s someone around here who knows an awful lot more about this subject than I do.