According to a hadith, you must desire for others what you desire for yourself. (Musnad Ahmad 16655)
You should want for your brother what you want for yourself. This is a very simple criterion. Every person can judge himself on this basis. One should think: "Do I desire for others what I desire for myself?" If a person engages in negative propaganda against you and presents an incorrect picture of yours before others, you would never like that. You would never want a person to spread a wrong image of yours among people. So, you must also not do this. You must speak the truth. You must not indulge in the habit of character assassination. If someone indulges in character assassination against you, you must never do the same. This is a very simple formula. In Persian it is expressed thus: "Do not wish for others what you do not wish for yourself." This is indeed a very simple and easy formula.