The thing is, as a proposition the devil is pretty much unlovable. And by "unlovable," I don't mean "difficult to love," I mean, "impossible to love, since it's not something for which the good can properly be willed."
You can't love evil as such, properly speaking. It's an oxymoron to desire the negation of something for the sake of that thing.
There is no personal good in the devil -- it is not a creature capable of goodness -- and there is no hope for personal good in the devil in the future.
The one thing about the devil that is good is its existence, because existence is always and everywhere a grace of God, a participation in the being of He Who Is. But the devil's existence is not something from which more good can come, which is why St. Thomas teaches that demons are not fit objects of charity in the way we usually think. No good exists which we can desire for the devil beyond the atomic fact of its continued existence.
Or is there? We should love all things with God's love, and it's possible (as far as I know) that the devil's eternal punishment is somewhat mitigated by God's mercy. We ought to desire for the devil whatever mercy God shows it, no more and no less.
So I might say, "We should desire the devil's continued existence and the precise amount of mitigation of its eternal punishment God has decreed." But "to desire continued existence and the precise amount of mitigation of eternal punishment God has decreed" is a somewhat obscure meaning of "to love" -- try it out on your sweet baboo next time and see what happens -- and as Mark Shea points out it may not be too prudent to go about saying, "We should love the devil."
If any use can be drawn from all this, it might be by making the point in negative terms: We should not desire the end of the devil's existence, nor more or less mercy toward it than God has decreed. In short, we should desire what God desires, even with respect to the devil.