"Hate" is a definite word, so the meaning is very clear and strong.
"Don't like" is simply the negative of "like". It's not necessarily hate as such - it's a softer version, and still tied to "like". It could equally mean you don't care about it. However you feel about it, it's not "like". Far less committed than "hate".
Another example is the reply to "how are you?" If you reply "good", then that's pretty clear. If you reply "not bad" it's not automatically good, but a less-committed positive answer. You may be OK, good, great, so-so but definitely not "bad".
In Orwell's novel "1984", the words "bad" and "evil" didn't exist in Newspeak - everything was described in relation to "good": good, plusgood, doubleplusgood vs. ungood, plusungood, doubleplusungood. So evil didn't have its own existence, it was simply described as something in relation to "good". Nasty piece of brainwashing, huh? :)