Ah ya i see your confusion. This is hard to explain to me i suspect someone has a better system or can explain easier. but from what i gather you want rates of kills.
lets say you want to know the kill rates over the course of the last 60 seconds at an update of 1 second per player. you will need a 2 dimension array of 50 * 15. so when player 3 makes a kill array [1-60] (that's 1 through 60) is increased by 1. stating in order every second and repeating at the end 1 box is emptied of kills, and the next box is displayed as to players as rank.