It will always come down to 2 possibilities.
Voljin and Saurfang.
Both of them promised when they were with Garrosh and both of them have the right to do it.
Saurfang was the mentor of Garrosh assigned by Thrall first sent them both to Northrend.
Voljin was the right hand and vice-warcheif assigned by Thrall too after he quit being warchief.
High Overlord Saurfang says: I won't let you take us down that dark path again, young Hellscream. I'll kill you myself before that day comes...Vision of Vol'jin says: And when tha time comes dat ya failure is complete and ya "power" is meaningless, I will be dere to end ya rule swiftly and silently.
Vision of Vol'jin says: Ya will spend ya reign glancin' over ya shoulda and fearin' tha shadows, for when tha time comes and ya blood be slowly drainin' out, ya will know exactly who fired da arrow dat pierced ya black heart.