Well, i thought you knew that:
The variable name is just "Kills". Then, in the window you create the variable, you need to tick that little thing called "Array". Once ticked, if you set the Kills variable, it will look like this: Kills[Index]. Press the "Index", scroll down to "Player - Player Number", press it, and then in the new window, scroll to "Owner of", and change "Triggering unit" to "Killing unit". That's it.