This tool tells you how many people you can kill unjustified without getting a red skull, an example is shown below. If you're having problems it's probably because you're using Internet Explorer, you need Firefox (or any other brower that tab delimits data when you copy it from tables).
You are currently below the unjustified player killing limits!
You can kill 1 player(s) right now without reaching red skull limits!
If you are red skulled and want your skull to leave, do not reach the red skull limits again!