I think he found out that there was a negative effect of killing La... So he decided to just kill Tui.

<p>I personally think he didn't really care about the Ocean Spirit. He killed the Moon Spirit because it would negate waterbending, giving the Fire Nation forces a huge advantage against the NWT. Thus he had a very specific goal in mind, and the Ocean Spirit was irrelevant to that goal. </p>

<p>Adding to what everybody else has said, I doubt that Zhao knew that Aang could merge with the ocean spirit to become some giant koi monster. But even if he did, we don't know what the consequences would be of killing La. </p>

<p>i agree that zhao probably knew if he killed atleast one the balance would be upset and the other would be pretty much useless rendering waterbenders useless.i dont think he decided not to kill the ocean spirit to avoid a water crisis .i dont think zhao could think that far ahead since 'killing' the moon would affect tides ,having similarly catastrophic effects.he wasnt really thinking of anything aside from wiping out waterbending </p>

Did he really know the difference between them?

<p>That's not entirely clear, I think. He may have known that the Moon Spirit was one of two, or he may not. All we know is that he specifically targeted the Moon Spirit alone. </p>

<p>i agree he may or may not have known .all he was probably thinking was <i>lolz ,killing a magic fish will make watermagicks die ,lmfao lmfao</i> </p>

<div class="quote"><i>Weltall8000 wrote:
Did he really know the difference between them?</i></div>
<p>He mentions in Siege of the North, Part 2 that the scrolls he found in the library had a detailed description of the spirits. I can't read Chinese so I don't know how detailed it was but it was apparently good enough for Zhao.
</p>

<p>I think he thought removing the moon wouldn't be as bad a removing the ocean. Think: the moon is where the waterbenders learned bending right? And the ocean did...what? Guess he mainly wanted to weaken waterbending. Besides, if killing the moon spirit destroyed the moon, killing her counterpart would pretty much dry up the ocean... </p>

<p>The Water Tribes derive their power from the moon, and their lives from the ocean. Zhao's goal was to conquer the Water Tribes, not eradicate them. He wanted more people that he could subjugate to the will of the Fire Lord. It's easy to defeat a people that has been disarmed of its greatest weapon (in this case, waterbending), but it's hard to rule over that same population once you've killed them all. Eliminating the moon spirit and sparing the ocean spirit ensures that the Water Tribes would remain alive to be subjects of the Fire Nation, but in no position to pose a real threat. </p>