Does Arthas deserve a "happy" ending?

Not quite. It’s Malfurion after all who ended the story in the RTS original incarnation of the franchise when he invokes the Wisps to detonate Nordrassil and kill Archimonde, ending the Third War.

Tyrande takes out Nathanos after literally pinning him to his cabin and disintegrating his dogs with Dark Moonfire.

(I really am pissed that when they finished the revamp for Hyjal, they removed his skeleton from the tree.)