He forfeits his title as Justice knowing that Justice will be unregulated in his absence giving rise to undead swarms and only a hero can stop it. Everything that happens is a self fulfilling prophecy conceived by Tyrael. Either he knew what would happen and became corrupt, or he honestly had no idea and still killed countless numbers of mortals, brought war and torment to Sanctuary to end a problem he creates in acts 1-3, if that's the case he doesn't sound very wise.
Now that the Nephalem know of Zulle's work and that Aurielle holds their hope the attack plan is simple. Kill her, take the war to the council.
Get Lillith to assist the Nephalem by getting her to send demons somewhere, ask if you can talk to the council in Heaven to form a strategy against her and betray them then. After the council is destroyed Lillith will be killed as well. Sanctuary will no longer be controlled by idiots and mortals can actually claim free-will.
After that, please stop making Diablo games.