SolidSnake14, the first argument in onPlayerWasted event function is ammo, apparently you forgot this :b
function onWasted(ammo,killer)
if getElementType(killer) == "player" and killer ~= source then
outputChatBox(getPlayerName(killer).. " killed ".. getPlayerName(source) .."!",getRootElement(),255,0,0)
end
end
addEventHandler("onPlayerWasted",getRootElement(),onWasted)