Vaiṣṇava-aparādha generally takes place due to the presence of īrṣā and bhaya, enviousness and fear. When this aparādha is committed, krodha, anger, will arise. The offender will become very angry at a particular vaiṣṇava. He is afraid of that vaiṣṇava because of that vaiṣṇava’s great power. “He is making rapid advancement and I am not. Most people will be attracted to him and no one will be attracted to me.” Due to this envy and fear he finds fault with the vaiṣṇava, criticizes him, and speaks ill of him. Hatred and criticism arise and he speaks ill of that vaiṣṇava and concocts stories about him. Thus he commits vaiṣṇava-aparādha.
We often tell the story of the wolf and the lamb. “Somehow or other give him a bad name and kill him.”
A wolf was drinking water from a river at the same time as an innocent lamb. The wolf thought, “Somehow or other I must devour that lamb.” The wolf said, “Hey! Don’t you know? Can’t you see that I am drinking water here? Why are you making the water muddy for me? I shall kill you.”
The lamb said, “Oh sir, you are drinking water upstream and I am drinking water down-stream. How is it that I am making the water muddy for you?”
“Why did you call me bad names one year ago?”
“Oh sir, I am only three months old. How is it that I called you bad names one year ago?”
“Your mother must have done so. Anyway, I will kill you.”
This story of the wolf and the lamb illustrates how hatred, speaking ill, and concocting stories arise from intolerance or enviousness. You may say, “This devotee is not chanting his rounds! He is not following the regulative principles! He has become a vaiṣṇava, but now he is engaged in illegal and evil activities! If I remain silent and don’t speak about it then I am giving him the opportunity to commit more and more evil activities! I will be indirectly supporting him, just as if I would say, ‘Yes, do it!’ So in order to do good to him I shall speak to him.”
Now if one subtly analyses such things one can understand that you are simply finding fault with that devotee. But are you perfect yourself? Are there no defects in you? The saying is there, a sieve has many holes and a needle has only one hole. The sieve says, “Oh needle, there is a hole in your back.” The needle replies, “You have so many holes. I have only one hole.” But the sieve criticizes the needle. So, one who criticizes should understand whether he himself is perfect or not. Are there no defects in me?” Only if someone is perfect can he find fault with others.
In the śāstras and in the Bhāgavata it is said that this nindā-karaṇa, criticizing and finding fault with others, is a great anartha. Don’t do it! It is not your lookout. That is guru’s business, it is his lookout. The sages say, “Only see the good in others. Don’t see the bad. If you have the nature or tendency to see bad, then see what bad is there in you.” This is āpta-vacana, authoritative instruction: “If you find something good in some person, then that is what you should accept. Close your eyes to the bad things and open your eyes to the good.”
There is one story in this regard. Once a disciple of Śrīla Bhaktisiddhānta Sarasvatī Mahārāja came to him and spoke ill of an-other disciple. “Mahārāja, Mr. X is doing this thing and that thing.” For half an hour he spoke all bad things about that other disciple.
With much patience, Bhaktisiddhānta Sarasvatī listened. Then he said, “All right, you have spoken all bad things about him. Is there nothing good in him?”
“Yes, Mahārāja. He sings very nice kīrtana.”
“Then why don’t you speak that thing? You spoke all bad of him, but you never said that he sings nice kīrtana.”
To find fault or criticize some vaiṣṇava is not good at all. Doing so you will commit aparādha and that is a great anartha. Only someone who is perfect can find fault and correct another person. He has the right to be critical because he is perfect and has no defects in himself. If you have some defects, then why are you seeing the faults in others and speaking about it? Thus, the ācāryas, sādhus, and śāstras have forbidden it.

— Śrīla Gaura Govinda Svāmi Mahārāja (Kṛṣṇa Kathāmṛta Bindu Issue 61)