And also considering that the only sheikah we have an example of is essentially freakishly loyal to Zelda. I think we'd all agree Impa would risk her life for the Royal Family. Of course that's not definite proof, she could just be an exception and the Sheikah in general were traitorous, but it seems unlikely.
Also, considering the Shadow Temple, I think there are a mix of reasons why it is so hellish. One, I think its a game element, not a story element. It's the shadow temple. They need it to be evil. They weren't actually designing it as a place for worship, they were designing it as a level for the game, so that needs to be considered (aka, what exactly "canon" means. Second, it exists in Gannondorfs perverted version of Hyrule, so it could have become considerably more evil under his reign. Thirdly, even if it does seem "hellish" and "shadowy" that same thing applies to most chthonic religions. That doesn't mean they're necessarily evil, just different than the culture of the current world's predominant monotheistic religions. So don't necessarily let that bias you.
On the other hand, I can't prove that they Sheikah WERENT traitors, they very well could be. I'm just saying that you shouldn't at all make that assumption based off the canon.