I think you're absolutely right about the negative power that ideology can have in terms of getting (some) people to say that the ends justify means, no matter

I think you're absolutely right about the negative power that ideology can have in terms of getting (some) people to say that the ends justify means, no matter how atrocious. However, I wouldn't say it's the only thing that can cause people to defend the indefensible. Indeed, for some, ideology itself in the means to an end, and these people can be quite flexible about how seriously they take it depending on how well it supports their more fundamental goal, which is raw dominance.