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The Gemora states: If he calls him a evil person, he (the insulted person) may descend against his life (he is permitted to hate him to such a extent that he may attempt to reduce his income).
Rashi explains this to mean that the insulted person may fight with him as if the libeler hit him, and it is as if he was coming to kill him. Furthermore, Rashi heard that he can compete against him in his line of business in an attempt to decrease his income.
Rashi asks that it is hard to understand how the Chachamim would allow this person to take revenge.
Some answer that here it is permitted because he suffered personally and he was subject to a public humiliation. The Chafetz Chaim, however, writes that it is unclear if this is the accepted halachah, and therefore, one should be stringent in the matter and not take revenge.
Others answer that it is permitted because if people think that he is indeed a evil person, his income will suffer tremendously, for people will not have compassion on him.
Tosfos writes in the name of the Gaonim that it is permitted to burn one-third of his grain. Tosfos concludes that this is bewildering, for where is the source for this?