Well I don't know that good motives always have to have good results, but you do want there to be a sense of proportion and good qualities and traits and motivations being recognized in the text and if possible rewarded.

I thought the Cedric death worked partly because it was really shocking. You're uplifted with the characters thinking they are winning this tournament together, and then in a moment he's dead and everything changes for Harry and for his world. But - I don't know - Harry doesn't learn anything from this event. And he was so reckless and even a bit corrupt in the way he went about the tournament, cheating and lying and taking advice from the corrupt impostor. So what was the point of it all? It shows something horrible happening, but there is no moral issue or lesson here, just Harry being a victim, so we sympathize with him.

I mean, the Snape death would work better if it was clear that somebody learned something from it or changed as a result or, I don't know, someone was just really shocked and horrified. And she could have ended the saga in a bittersweet way, instead of this confused tragic/happy ending. Plus you're left wondering what her intentions were with the Snape character, because the book - and author - seem very conflicted about him.