I think you're confused about what a library is. A library is code that is shared between addons, such as LibSink-2.0, which handles the specifics of outputting text to many different combat text addons so that individual addons wanting to display text messages don't have to write all of that code themselves.
I think the terms you're looking for are "variable" and "table". You'd probably want to store the name of each killing blow source or recipient (I'm not really clear on whether you want to record how many killing blows you get on others, or how many killing blows others get on whoever) as keys in a table, and increment the value each time another killing blow for that source/recipient is detected. i.e.: