Nazgûl Theme

[piosenniel]

Since your question is:

Who was responsible for the destruction of the Lord of the Nazgûl

I would have to say that the 'responsibility' lies with both Eowyn and Merry. Merry's strike with the blade of Westernesse allowed Eowyn the opportunity to strike the fatal blow.

If you had asked who 'killed' The Lord of the Nazgul, I would have to say that it was Eowyn who gave him that mortal blow as foretold in his prophecy.