Garrosh won't kill Aggra, yet he will be partly responsible for her death.
In order to keep the Horde going against Humans, a member of the alliance
has to kill Aggra while trying to kill/stop Garrosh. That would keep the WAR
back in Warcraft still and make Thrall kill all those involved especially Garrosh
who started it all.
My proposal is that in the Org Raid or the patch before it, that Jaina will
go to kill everyone in Org again or kill Garrosh while innocents are around
then Aggra stops them with her Sha-ladyness. Jaina kills her because of
her great power and ... because Jaina is crazy. This makes Thrall mad at
both Garrosh who started this conflict and at humans who just want to blindly
kill orcs.