The Prophet ﷺ once said:

"Do you know what backbiting is?" They said, "Allah and His Messenger know best." He said, "It is to mention about your brother that which he dislikes." Someone asked, "What if what I say is true?" The Prophet said, "If what you say about him is true, then you have backbitten him; and if it is not true, then you have slandered him."

You cannot justify backbiting by saying it is true. Truth spoken with the intention to harm is still a weapon. The Prophet ﷺ made this crystal clear.