It's definatly a mutual respect thing, they're both hardcore killers, both to that point unbeaten, then finally they face someone who doesn't just DIE, so they both have a wary respect for one another.
Unohana as Zaraki's mum? God I hope not
Why does somebody have to die for Zaraki to be stopped? (and why would be it Yachiru :S), I'm guessing Zaraki will be about to lay down a killing blow, then SOMETHING will happen, either the Stern Ritter will attack, or someone will burst in to stop that happening. Just doesn't seem right to kill off Unohana like this.