Onslaught is not a physical being so Superman can hit him all he wants he can't kill him. It's even safe to say that Onslaught still exists just not in the form we know, if he is a manifestation of the dark sides of Xaiver and Magneto we can assume he was split back up into his native forms and reabsorbed, because these two still have dark sides.
http://upload.wikimedia.org/wikipedia/en/1/1f/Onslaught_01.jpg pretty sweet pic I think..
