Because he's a hypocrite. He's all for forcing his "destiny" on others but refuses to have a destiny forced on him.
They also just wanted to make the light 'evil' so they can then have the Orcs play victim again in the Mag'har race intro. What Blizz has done to the Light as a force for good in the Warcraft universe is terrible, turned it into some GoT knockoff where everyone has to be a secret Hitler.