I'm sorry if someone had already mentioned it here, but I haven't found it in the comments. In my opinion this is a quite clear indication of how Gandalf feels about the Witch Kings power.
“My friends,' he said, 'and all you people of this city and of the Western lands! Things of great sorrow and renown have come to pass. Shall we weep or be glad? Beyond hope the Captain of our foes has been destroyed, and you have heard the echo of his last despair. But he has not gone without woe and bitter loss. And that I might have averted but for the madness of Denethor.”
