[Gurthang]

Maybe Tolkien was trying to make a point with these opposite powers: that the power to build up is greater than the power to destroy.

Look at the rings themselves. If you say they were completely opposite, then they should counter-balance each other. The Elven Rings are three, but the Nazgul's are obviously nine. This would mean that Sauron had to make three 'negative' rings to each one of the more powerful elven rings.

This could also elude to the power of good being a greater force than the power of evil, even though evil seems to way outnumber the good. As in the case of Mordor vs. Gondor/Rohan.