Justice droids were used years before the Clone Wars, and were so successful that they were reproduced by the Galactic Empire during the New Order. They were both strong and durable, and thus reliable to use.
One of these droids was the Imperial assassin droid 2-124, that was captured by the Rebel Alliance. The droid escaped and killed a number of Rebel and New Republic officials over the years. It was placed in General Airen Cracken's confidential New Republic Intelligence brief listing the top fifty most dangerous beings to the New Republic.