I think the prime evils wanted their soulstones destroyed so that they could return to hell and reclaim their throne. Before they returned to hell they needed to soften it up. What better way than to destroy the worldstone, unleashing a holy/unholy war between heaven and hell. When Belial and Azmodan were all but defeated they would make their return and become rulers of hell once again. I believe Tyrael tried to stop them but was overcome by corruption in the presence of two prime evils within tal rasha's tomb. He pretended to befriend you and led you to all of the prime evils and their soulstones in order to break them from the mortal realm. After killing baal Tyrael destroys the worldstone in order to set the plan into action. Tyrael is soon after banished to an icy tomb to spend all of eternity in a frozen sadness by the agrinis council.
thats easily the most awesome theory im yet to hear on diablo 3 ever
yeah but if he's right itll be a thousand times more awesome
thats easily the most awesome theory im yet to hear on diablo 3 ever