DiabloFans

Quote from Aerisot

You didn't play the entire game did you.

Edit* Also he is not an "Aspect" That is WoW, Tyrael WAS the Archangel of Justice, by the end of the game he now stands as Wisdom

Quote from TheDFO

It wasn't Izual's trick, unless he turned to the Demon's side before he got captured.

The greatest of these heroes was Izual, lieutenant to the Arch-Angel Tyrael and bearer of the Angelic Runeblade Azurewrath. He once led a fierce attack upon the Hellforge as the creation of the dark demonblade Shadowfang was nearing completion. His quest was to destroy both wielder and weapon - a charge that he was destined never to complete. Izual was overcome by the legions of chaos and, tragically, was lost to the Darkness. His fate stands as testament to the fact that Angels and Demons alike shall fearlessly enter into any domain - so long as their hated enemies dwell within.

Tyrael: Despite his valor and strength, Izual was captured by the Prime Evils and twisted by their pervese power. They forced him to betray his own kind and give up Heaven's most guarded secrets.
He became a corrupt shadow of his former self - a fallen angel trusted neither by Heaven nor Hell.

Izual: Tyrael was a fool to have trusted me! You see, it was I who told Diablo and his Brothers about the Soulstones and how to corrupt them. It was I who helped the Prime Evils mastermind their own exile to your world. The plan we set in motion so long ago cannot be stopped by any mortal agency. Hell, itself, is poised to spill forth into your world like a tidal wave of blood and nightmares.
You and all your kind... are doomed.

Tyrael: Thank you, hero, for putting Izual's tortured spirit to rest. May the Light protect you and the powers of Heaven shine upon your path. But, if what you tell me is true, then I fear that we have been played for fools all along. Izual helped Diablo and his Brothers trick me into using the Soulstones against them... Now the Stones' powers are corrupted. With the combined powers of the Soulstones under their control, the Prime Evils will be able to turn the mortal world into a permanent outpost of Hell!

Ithereal: The angel Izual was once Tyrael's lieutenant, but he was captured by the enemy after an ill-advised assault upon the Hellforge. Izual surrendered to demonic corruption and revealed the secrets of the soulstones to the Prime Evils. Tyrael claimed that Izual was slain years ago, but his replacement has never emerged from the Crystal Arch...

Which points to Tyreal was either only half informed, or foolishly arrogant, about the soulstones. If he knew that these stones wouldn't actually allow them to corrupt humans and gain stronger bodies, I doubt he would have used them.
...
That said, I agree that we should not assume whatever he says is correct. He thought the Soulstones could protect them, they couldn't. When he found out that the stones could be corrupted, he...waits until the Wanderer almost frees Baal? One would think he would have checked up on those stones, at least once a century or something.

Deckard Cain:The time has come for you to destroy Mephisto's Soulstone!
Take the Stone to the Hellforge. Place it upon the forge and strike it soundly with the Hammer.
Only by doing this can you prevent Mephisto from manifesting in this world ever again.

Then after you beat the Terror's End quest:
Tyrael: Praise be to the Light! You have accomplished the impossible!
Diablo and Mephisto have been banished back into the Black Abyss that spawned them and the corrupted Soulstones are no more.
You've done well, hero. For now, you should rejoice.

No longer in command of his body or spirit, he raised the stone above his head and uttered words long forgotten in the realm of mortals. His will destroyed, Lazarus shattered the Soulstone upon the ground. Diablo once again came into the world of Man. Although he was released from his imprisonment within the Soulstone, the Lord of Terror was still greatly weakened from his long sleep and required an anchor to the world. Once he had found a mortal form to wear, he could begin to reclaim his vastly depleted power.

The Nature of the Soulstones
by Jered Cain of the Horadrim

It was long ago that the enigmatic Arch-Angel Tyrael bestowed upon us the secrets of the mysterious Soulstones. Tyrael bequested upon our Order three of these Stones so we could contain the vile essences of the Three Prime Evils who had been let loose upon our world. Although the artifacts were constructed in realms far removed from our own, we found that they were simple to understand.

The Soulstones effect only beings that are non-corporeal and thus have no power over living, breathing creatures. When invoked, the Soulstones bring into being a strong “spiritual” vacuum. Any non-physical entities caught within this vacuum are drawn into the burning recesses of the Soulstone and are forever trapped within. These spirits are released only when the Soulstone is deactivated or destroyed.

The power of the Soulstones proved to be much more difficult to employ when used against the great Prime Evils. Voraciously disposed to possessing hapless mortals, the Three Brothers found that they were immune to the effects of the Stones while occupying human souls. Sadly, we were forced to hunt down and kill the innocent victims of the Prime Evils so that their demonic essences could be subject to the effects of the Soulstones.

Mephisto and Diablo, once found, were easily lured into the Soulstones. The capture of their brother Baal, however, became complicated when the Soulstone that was to be his eternal prison was shattered and fragmented. We found that while the shards still held the power to lure the demon to them, they could not properly contain it. Tal Rasha, a fellow Initiate who has been since immortalized in Horadrim lore, theorized that a mortal of strong will might be able to contain Baal within his own mortal soul. This sacrifice meant that the essence of any mortal so chosen would be forever tortured while locked in eternal conflict with the enthralled demon. To this end, Tal Rasha volunteered to contain the raging Lord of Destruction.

Piercing his breast with a shard of the Soulstone, Tal Rasha took within himself the essence of Baal, the Lord of Destruction. The Initiate’s body was shackled, chained and buried deep within a tomb under the desert. The sacrifice of Tal Rasha has kept Baal imprisoned for many years now, and although the demon was imprisoned without the use a whole Soulstone, we believe that our victory may be a hollow one. Should Tal Rasha ever escape, he would have the formidable powers of Baal added to his own. By ridding the world of this present Evil, we may have created a nightmare worse than that which we first sought to contain...