Resentment, and perhaps more generally hatred, is a valuable feeling that is treated unfairly in the present. Hatred, particularly visceral, is a survival trait and yet is often marked as undesirable. In the context of societal structuring, hatred serves as protection against subversion and the like.
It's common nowadays for one to ask for reasons for hatred, but this doesn't usually mean what would seem. Typically when one asks for reasons, one merely seeks to dismantle the hatred by knocking out support; that's to write one asks for reasons usually so they can be deemed unreasonable and attempt to stop the hatred. The goal isn't to understand the hatred, but merely stomp it out, unreasonably.
Another utility of hatred is self-improvement, as there's excellent motivation in seeing a loathsome individual or act and seeking to pursue the opposite. Situations improve when a man hates something so much so as to fight to remove or replace it. It's clear that hatred is a fundamental motivation.
Clearly, not all unreasonable hatred is good or encouraging improvement, but to compare it to poison is foolish; unreasonable and visceral are often conflated, but visceral hatred is necessary to live. As a man with more than mere reason at my disposal, I refuse to discard all other forms of decision.
The goal of making a less hateful group is of making a group weaker, and thus more easily subverted.