...kill Sophie's son?
At one point, he even asked her if she thought he did it. That could explain her giving him the "water burial" after receiving the gift of forgiveness. Also, him dying (and presumably being cursed) would fulfill his promise to her that she would have vengeance.
Thoughts?
I don't think he had anything to do with her son's death.
As for the water burial, as you put it, to me it would just be a good way to get rid of the body, otherwise she would have a lot of explaining to do. The guy seemed to be a loner, so probably no one would know he was there with her without a body.
And I believe that she just let go off trying to know who killled her son, "the gift of forgiveness".
I had the same questions although I came to the conclusion that she changed her mind about what she wanted. But this begs the question, did she lie? He kept berating her for lying ('a half truth is a lie') so if she changes her mind about what she wants from the angel, is it a half-truth or a lie?