Now, to clear up some confusion in episode 3. When Mao asks where Yin is, Hei replies with "Yin... I... I will kill 'her.'". It's hard to translate the first line when Hei doesn't use any verbs. Some ideas as to what he might have meant may be:
"I left Yin to die."
or
"I abandoned Yin."
Something along those lines, though I can't be sure of course. But this means that the person he means to kill doesn't exactly mean that it's Yin, but most likely the person who caused Yin to "die", be taken away from Hei or cause whatever happened to her. The best answer to this though is to wait and continue watching DtB as the story unfolds, which I'm sure you all will.