Kurtz' final utterance - What is "the horror" to which he refers, and why are these words the final words he speaks?

I think this statement is ambiguous at best. I think we can infer that he has either,

1. Kurtz sees his life pass before his eyes, as the dying sometimes do, and realizes the terrible things he has done.

2.Kurtz has a vision of his afterlife which pretty involves a hell of some sort.

In any case we know that Kurtz did not die a content man.