Well, part of my first answer to this question was that some people are so set in their hatred that even when they gain understanding, it doesn't help to lessen their hatred. In a case like that, it could involve anything. It can be about something complicated--such as a hatred for something religious/political--or it could be for something as simple as the opinion on a book or movie.
Wow. That was not as eloquent as I wanted it to be.
Here's another try. I think a situation where gaining understanding doesn't lessen hatred could be with something such as rape. The victim might come to understand why they were attacked--that rapists do it to feel in control--but that wouldn't exactly lessen their hatred for the rapist. I don't think it does for many people. It might just make them feel even more disgusted.
Another example could be murder, whether it's murdering one person or many people. With things such as these, I don't know whether we should feel less hatred towards the people. Does it help to understand their mindset? The situation they were in? Maybe. Maybe not. It's debatable, really, but I don't think it should make us feel indifferent. I don't think we should feel pity or sympathy for them, either.
This question reminds me of a project my younger sister just did recently. She was doing research for the Virginia Tech masscre and ended up finding it a lot more interesting than she thought she would. In the media, it's presented one way. The gunman entered the campus and eventually ended up killing thirty or so people and then himself. That's what she got at first. But then she went on to read about *why* he did what he did, and eventually, that made her feel a little pity for him. In a situation like this, it did lessen her hatred, but she also went on to read the comments people had written about the massacre, and even though these people knew the same thing she knew, they still hated this man very much. Why shouldn't they, after all? It doesn't matter that he was bullied all his life. He still took the lives of thirty-three people who didn't deserve to die based on what had happened to him.
I also read an article about the genocide in Darfur and even though I got a glimpse into why the leader was so damn cold, it didn't make me hate him any less. If anything, it made me hate him even more and I wished, for a second, that I hadn't gotten to learn about why those people (I'm including his so-called "soldiers) were doing the things they are. Why? Simply because there's no good reason to kill an innocent person, let alone trying to wipe out people in such great numbers.
There are plenty of examples in history where gaining an understanding of the situation doesn't make you hate what happened, or who did it, any less. There are two sides to every story. Just because you've heard both sides of the story, it doesn't mean you'll dislike what happened (or the people involved) any less.
As for your second question, yeah, I think many people keep themselves in the dark to continue hating. The first examples that come to mind involve religion. Look at the Religion and Spirituality section. That's a prime example. People don't want to listen to each other or delve deeper into things because they want to continue hating each other. (That includes both sides, by the way.)
Some people believe in certain things to the point of hating what opposes their beliefs. Those people end up going out of their to avoid understanding the opposite side because they're afraid by listening, they might switch sides. But there are many reasons for why people avoid gaining understanding.
I believe a lot of it has to do with fear.
Uh...I hope this answered your question. It's a great question, by the way!
**I really liked Joshms example. I think that was a perfect example in which gaining understanding doesn't lessen hatred.