The thing is, your basis that fiends cause harm by their very presence is wrong, except on the material plane. There, and only there, do fiends cause harm by their very presence.

Killing a fiend is always good, but it's also evil if that particular fiend doesn't prey on the innocent, and there is nothing inherent to their nature that requires that to be so, as long as they avoid long-term presence on the material plane.

Now, if you kill fiends because fiends are evil, you're gonna be right almost every time. You may spend lifetimes killing them without ever murdering, because you might not run into one that doesn't prey on the innocent. But, in the ridiculously rare event that you happen to run into such a fiend, and kill it, because 'fiends are evil', you've committed murder. So, if a paladin did that, he'd fall. Because murder is evil, and paladins fall when they willingly commit any evil act, no matter how balanced it is. The paladin could probably get an atonement, and it likely wouldn't be too huge a deal. Maybe he'd have to get a True Resurrection for the fiend he killed to make up for it. But it'd still have been an evil act.