[Spoilers ahead] I feel this may be an unpopular opinion, but when I read through ROW and got to the part where our man Taravangian ascends to Odium, I didn't feel the threat of it.

Taravangian might have been a 'ends justify the means' sort of guy, but his 'ends' have always been that he wanted to 'save humankind'. Sure, his plans conflicted with Dalinar, but that doesn't make them inherently bad. (Although they were, quite often, morally corrupt due to him thinking of them on 'brilliant' days).

In any case, there's now a man in control of the shard which orders about the enemy army, who doesn't even want the enemy army to win. How is this a losing situation? Dalinar might lose, but humanity on Roshar won't. What would Taravangian gain by continuing the war there? How do Taravangian's displayed motivations even align with crushing the humans and making the singers win? There's no indications he was ever fighting for this. In fact, all we got to learn of him in ROW is that he wanted Rayse/Odium gone and dead. So now that he is Odium, what makes us think that he's going to continue the reign of terror which he was so desperately trying to stop? I don't buy it.

When I read the 'save them all' line, I legitimately read it as a happy ending. Like "oh, that's nice...what the hell is the point of the next book, then?". The humans won. A human became the enemy god. War over, right? I don't even see tricking Hoid as a bad thing. Hoid is a meddler and an eternal, cryptic jackass. I trust his plans less than I trust Taravangian's.

So...why am I wrong? Why is Taravangian with God Powers a threat?

EDIT: Lots of great answers below. I'm convinced not only that Hoid is probably meddling the right way, but that Taravangian and Odium's idea of 'saving them all' is pretty well aligned. Nice work Stormlight community.