Step-by-step explanation:
Lex talionis means treating the criminals in the way the criminals treated the victims. In means giving the same sufferings to others as others have caused them suffering.
This principle is not plausible because according to Nathanson, it will make us to act in an immoral way with others. We cannot hijack the hijacker's plane because they did to us. We cannot spy on others because others are spying on us. Nathanson objected the principle because he believes that it is not possible to measure sufferings of others as different people suffered differently. So we cannot bring equal suffering.