Because saints know that their killers are killing their own souls, not their souls. Their body’s painful death is inevitable and that is nothing compared to killers’ souls’ (personality’s) eternal death and then eternal torment of soul parts. But spiritually blind humanity thinking body as self takes body death as ultimate death. To save the body they kill the real self, their soul, by using soul-disintegrating hate-violence. When one cultures hate he destroys his internal love-bond to disintegrate his soul made of many parts bonded together by love. So Christ said that sword wielder will be destroyed by the sword.
Some may object that many such killers were never killed by sword. No! It means that the internal hate generated by the sword is sure to destroy the soul in time. As spiritually blind humanity see only the body, they fail to see the destruction of the soul. But we see often that criminals’ personalities are destroyed soon if not killed by their likes beforehand. But great criminals often defy personality death due to empowerment by rebel angels (Devils). So we see mass killer emperors, war lords and despots maintain their integrity as long they live. Devils inherit from heaven very strong soul integrity which they use to destroy others and themselves too in the process. It takes much longer than weak humans to break apart by hate-violence works. Devils give their weak human protegees their support to keep their soul (personality) together as long as they can use them. But when their usefulness is gone they withdraw their support instantly and their souls burst apart. They often commit suicide due to depression as Judas, Hitler etc. did. Often this is made to happen after death out of the sight of humanity as devils fear that if it happens before death people may see the truth and flee from Devil.
But saints can’t be deceived by Devils. They reject their allurements of worldly riches like Christ, not to be their agents of destruction. Devils pay worldly riches most to their agents, destroying mass morally and physically.