Both of them are sinful. One must not defend himself by backbiting others before verifying the truth or facing that individual. You can say the truth without backbiting him.

if a muslim backbites a muslim and the other muslim also backbite him by defending himself, who will get punished on the day jugement? Sometimes people make up lies about you and you respond by saying they are liars for example.

Backbiting | November 12, 2015 3:00 am
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If you backbited somone but then after said somthing good about him with intention to cancel it out, is it still sin ?
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People often gossip about celebrities. Some of these celebrities do very bad things. I am wondering if the celebrities get the good deeds of those who backbite against them, even if the celebrities are doing immoral things.