"What kind of debate is for the sake of Heaven? The debate between Hillel and Shammai. And what kind of debate is not for the sake of Heaven? The debate of Korach and his crew."
The idea here is this: We see many antagonists standing against the tzaddikim, and speaking about them with arrogance and disdain. So how can we tell if their intention is to critique and disparage true tzaddikim, or if their intention is pure [for the sake of Heaven], as many of them will tell you? There are even great Torah scholars involved in this debate, who say their intention is for the sake of Heaven.
This is how we can tell if they speak the truth, that their intention is for the sake of Heaven: If their fellowship is harmonious, if they are all friends in every way, then we know that their intention is also truly for the sake of Heaven. For this is the way of true and complete tzaddikim - their souls are all bound together in great love and affection, and they love their friends more than they love themselves. And no one is jealous of his brother, and when he sees his friend's greatness, his soul is happy and his heart rejoices, and he hopes his friend will become greater and greater.
But when we see these antagonists who have no spirit of friendship, but are all just on fire with debate. But they have no desire for friendship, and their hearts are separate and divided, and no one of them recognizes the greatness of his friend, and what they really desire is what their friend has, whether it is money or greatness, this is a sign that a bitter fire rages in their camp, and it is nothing but wickedness in their hearts that causes them to be jealous of their fellow, and to speak against the tzaddikim, who are the true servants of God.