He was rather an egomaniacal asshole, too. He became infamous for a quote, evil when taken out of context and incomprehensible when quoted in full, about Sept. 11 being a great work of Art.
Right, his statement seems to have been taken out of context. He meant it was Lucifer's greatest work of art. Here's what Wiki has to say about it:
Controversy
In a press conference in Hamburg on 16 September 2001, Stockhausen was asked by a journalist whether the characters in Licht were for him "merely some figures out of a common cultural history" or rather "material appearances". The composer replied, "I pray daily to Michael, but not to Lucifer. I have renounced him. But he is very much present, like in New York recently" (Stockhausen 2002, 76). The same journalist then asked how the events of September 11th had affected him, and how he viewed reports of the attack in connection with the harmony of humanity represented in Hymnen. He answered:
Well, what happened there is, of course"”now all of you must adjust your brains"”the biggest work of art there has ever been. The fact that spirits achieve with one act something which we in music could never dream of, that people practise ten years madly, fanatically for a concert. And then die. [Hesitantly.] And that is the greatest work of art that exists for the whole Cosmos. Just imagine what happened there. There are people who are so concentrated on this single performance, and then five thousand people are driven to Resurrection. In one moment. I couldn't do that. Compared to that, we are nothing, as composers. [...] It is a crime, you know of course, because the people did not agree to it. They did not come to the "concert". That is obvious. And nobody had told them: "You could be killed in the process." (Stockhausen 2002, 76-77.)
(To see how the excerpt appeared out of its context, and in English translation, see Tommasini 2001.)
As a result of the reaction to the press report of Stockhausen's comments, a four-day festival of his work in Hamburg was canceled. In addition, his pianist daughter announced to the press that she would no longer appear under the name "Stockhausen" (Lentricchia and McAuliffe 2003, 7).
In a subsequent message, he stated that the press had published "false, defamatory reports" about his comments, and clarified as follows:
At the press conference in Hamburg, I was asked if Michael, Eve and Lucifer were historical figures of the past and I answered that they exist now, for example Lucifer in New York. In my work, I have defined Lucifer as the cosmic spirit of rebellion, of anarchy. He uses his high degree of intelligence to destroy creation. He does not know love. After further questions about the events in America, I said that such a plan appeared to be Lucifer's greatest work of art. Of course I used the designation "work of art" to mean the work of destruction personified in Lucifer. In the context of my other comments this was unequivocal. (Stockhausen 2001a)