You could not have known the extent of what he was doing.

[Nobody could have. Not with the Dark Side clouding everything at that time. Yoda had spoken on it more than once.]

And you're right. Ambition is dangerous. And you're right. No one person should hold even a tenth that much power. But here, that's less of an issue. Here, we need to be here for Anakin. Maybe, if we're lucky and the Force is with us, we'll find a way to change his future.