That's a good question, I was just thinking about that myself when I watched KB again the other day.
Not much is explained verbally, but much is shown, emotionally. I don't think Bill actually
did anything to
Hanzo, so much as Hanzo was devastated by what a dishonorable
RAT
Bill turned out to be. It's pretty clear that Hanzo has come to face a very personal issue with violence, and he is torn over the fact that the swords he makes so expertly (now kept only as "art" and sentimental pieces) get used to take peoples' lives. He said he made a vow to God to never forge another weapon, a vow which he breaks in order to make a sword that will ultimately be used to kill Bill, among others along the way. Kiddo refers to Bill as a
"vermin"
whom Hanzo should feel some responsibility over, since Hanzo apparently must have been Bill's mentor/sensei, or at least some kind of contact, at some point. Hanzo is so disturbed by the thought of Bill, that he can't even speak his name; he can only write it in the grime on the windowwhich Kiddo seems to enjoy
rubbing out,
an obviously symbolic visual meta2000phor.
Even though we are never let in on the specifics of Bill & Hanzo's relationship, we don't really require the details. Enough is expressed through the information, hints & body language that we are given. It seems that perhaps Bill started out less of a "bad guy," and got more evil over time by the cruelty of his deeds, while perhaps Hanzo was less caring when he was younger, but as he matured, seems to have come to respect & cherish life more. It's unclear just how much Hanzo knows about Bill, but he is obviously aware of Bill's nasty reputation, in general, & is upset for ever having had anything to do with him.
"If you just talk, I find that your mouth comes out with stuff." ~ Karl Pilkington

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There is supposed to be an anime about Bill's life which will explain why Hanzo hates Bill.

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I think "hate" is not the right word to describe what Hanzo feels regarding Bill. I sensed regret, sadness, perhaps a sense of guilt or perhaps shame. I believe cccinema is pretty much on the money as to why Hanzo helps the Bride by breaking his vow and creating "something that kills people." If memory serves me, he tells her, "I do this because I am philosophically in sympathy with your quest," or words to that effect.
He doesn't "hate" Bill, but recognizes that ending his life would be best for the proper balance of the universe.

I don't think the movie needs to go into details of why Hanzo hates Bill, as it is apparent Bill is a nasty S.O.B. deserving of hate from anyone with a conscience.

In the martial arts, when a master trains a student, that student is forever the masters responsibility, because he tought him what he is now using with evil purpose. when the bride says one of Hanzo's students has done her wrong, and writes Bill - Hanzo knows he is held responsible, hence he must make that sword, something he said he would never do again.
I think the idea is a little twisted, like Hanzo should hunt him down, but it still works.

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I think it's more of what Bill (and Budd) did with the Hanzo sword that upset and made him angry.
"Tell me, Mr. Anderson, what good is a phone call, if you are unable to speak?" - Agent Smith