I once read an article in Readers Digest with the title "Why we gossip". I started reading, prepared for something which was probably going to point out all the flaws in those people who gossip, and how they can be rectified etc.
But the article took a very interesting turn. It said, (paraphrasing) without gossip, people in power will arrange everything in the general direction of usurping more power (e.g. dictators). And how gossip serves a very important evolutionary need (a powerful, guerilla style technique to destroy power-grabs). If you were under a dictator, openly criticizing them leaves you with a small chance of being murdered. Hence you will choose instead to gossip. Imagine if the dictator then tells all his subjects - "whatever you have heard about me is untrue, it is just some haters hating".
So there is another explanation for haters (and their apparent obsession) - they look around and see a lot of injustice. Usually they are powerless to do anything, and waiting until they achieve something in life might make the problem much worse before they can offer their viewpoint. So perhaps hating has an evolutionary benefit - it gives clues to non-haters to then go and do their own additional research. Sometimes it results in reducing the power of those who become so out of touch that they just casually dismiss all criticism as hating, much like Hillary thought everyone who doesn't agree with her is a deplorable person.