[All good arguments, except "practical" doesn't speak to Cas, not in a situation like this: no, as it is, all demons are evil and unless it's a mutually beneficial agreement to be made with the current king of Hell, there are no good reasons for a demon left standing.]
You seem to be under a... misconception.
[Suddenly, there is the unmistakable sound of something like thunder, a strike of lightning that illuminates the shape of wings on the building in front of them -- mixed with Castiel's human voice is another, one that should shake any demon.] I am an angel of the Lord. He created us as his soldiers. Whatever else I may be... that is still my true purpose. Your excuses mean little to me.