So what options do they have now? If you cannot see that killing him vindicates his prejudices because we shown him that we will continue to slaughter and impede troll kind no matter what. So he thinks to himself - I was right. Humans/Orcs/Pandaren their all the same they take and take and never consider us. While we think the same of him and the genocide we feel he has called against us.
In the end two wrongs don't make a right. He's wrong and we're wrong but we know better.
Sparing him forces him to realize that we're not all the same and there is another path. If he can some how be made to realize that even in his death throes that the path for his people is born in peace than maybe then 4.2 and 5.2 will be worth it.
That's what I want to see.
Still doesn't justify tying killing Zul with committing genocide. Zul has committed crimes against the Horde and Alliance and for that he must be brought to justice regardless of how well he can justify his own actions.
This does not mean that after we kill Zul (along with most of the Zandalari's upper leadership most likely) we can't spare the remaining Trolls. Perhaps a new leader rises up who is more amicable to the idea of peace. Perhaps we simply intimidate them into not crossing us again without butchering every man, woman, and child.