His role in the chaos was a pretty ignorant one; he didn't know exactly what the messages between Sophia and Harney contained, but the bloodshed was so horrific that Huck narrates, "I aint' gonna tell all that happened-it would make me sick again if I was to do that. I wished I hadn't ever come ashore that night to see such things. I ain't ever a-going to get shut of them-lots of times I dream about them." He regrets passing on the note, and realizes that maybe he should have "told her father about that paper and the curious way she acted."
What Huck isn't giving enought weight to is that these families have been in a feud for generations, and are always itching to find a something to shoot at each other for. Sophia and Harney would have eloped whether Huck was there or not, and the resulting chaos was inevitable. He gives his role in all of it too much weight, but it is understandable, since it was so traumatizing to him. It is natural to think of the ways it might have been prevented, and this is what Huck does.