Not to mention the whole reason Arthas could even touch Silvermoon was because Darkhan gave him the key to the elfgates. Darkhan is the entire reason Quel'thalas fell.
It's always been Kael'thas that destroyed the Sunwell. After Arthas rezzed Kel'thuzad, it basically made the sunwell a beacon of necromancy. This was slowly killing all the elves, so they had to use what remained of the artifacts that powered the elf gates to destroy the Sunwell.