Zeus is said to balance the scales. And this is what Achilles and Thetis ask of him, give Troy some sort of victory so the greeks will come running back to Achilles.
I think Achilles forgot you can't get something for nothing with the king of the gods, favors and Iou's do not always hold up. Zeus might've been lenient with letting Achilles get away with the plan until Patroclus killed Sarpedon, a son of Zeus, in Achilles name. I think it was then Zeus decided that Patroclus could not live till the end of the battle. And so in price of Achilles request, Patroclus was taken from him.














