Try this, and please read the comments in it:
addEventHandler("onZombieWasted",root, --Wait for a zombie to die
function(att) --when a zombie dies, get the attacker/killer
local kills = getElementData(att,"zombieKills") or 0 --get how much zombies they killed and if they have not killed any, then give them 0
setElementData(att,"zombieKills",kills) --set how much zombies they killed
end
)