Board Thread:Questions and Answers/@comment-6673698-20130324101714/@comment-7313057-20130412191515

You are correct he did kill Unohana, but Unohana could have easily killed him. Her final task was to push Zaraki to his limit so he could release his full powers since he sealed it away. Plus Unohana wanted to die because thru death is the only way she can pass on the title of Kenpachi to Zaraki.