I wouldn't count on any one specific character getting the honor. Take Arthas for example. Countless characters had a strong reason to kill him. Mograine. Sylvannas. Jaina to put down the monster that had once been her lover. And the only lore character that was present, excluding those inside Frostmourne itself, was Tirion, who didn't really have as much of personal vendetta against him as other characters.
Given all that's happend, it will either by the hands of the players that finish him, or probably voljin or varian, we'll have to see, but a character saying they'll kill him doesn't mean they will.
---------- Post added 2013-04-23 at 10:09 AM ----------
I bet Garrosh would be the one to kill himself. The Old God's power will overwhelm him and he will be destroyed by it.
Secondly, you are only "returned" if you do not want to be undead. Whether you wish to join the Forsaken is entirely up to you. Numerous undead have left the Forsaken with no penalties, e.g. those @ Agent Dawn. Some never joined in the first place, e.g. Lilian Voss.