It feels a bit gimmicky and silly, and there are so many weird concepts thrown at you, but it's a very fun ride.
Ichi is a killer. He says he doesn't like killing. Every time he kills some one he's crying. But people keep asking him to kill them, by asking him not to kill them.
Kakihara loves pain. He's searching for a way to die. He hears Ichi is planning to kill him. He's excited, he wants Ichi to kill him, but has to protect himself and stop Ichi.
And in between these two characters are a lot of people dying, a lot of blood, and a lot of insanity.