Leoric wasn't a random choice at all. If you go and read the lore, you'll find that Leoric was persuaded by Lazarus to make Tristram the new capital. Tristram is where Diablo's soulstone was kept, but Leoric did not know this. Lazarus, however, did know this, as he had been the custodian of Mephisto's soulstone. It was a setup, and the brothers worked to free each other.
Furthermore, the three each have host bases that have things in common. All of Diablo's have been of Leoric's blood, meaning royalty. All of Mephisto's have been of the Zakarum, originally a religion of light. All of Baal's (all one) have been of the Horadrim, an organization of great wisdom.
Terror from royalty, when royalty is meant to protect and to inspire through rule, not to terrorize through misrule. Hatred from religion, when religion is meant to unite the masses and lead to peace and acceptance, not to promote wanton bloodshed and human sacrifice. Destruction from wisdom, when wisdom is meant to create and preserve, not to be a tool for destruction. Each is a complete corruption of what the host should stand for, the direct antithesis. It's thematic.
The match for Tyrael would be Sin, not terror, as Justice can naturally cause terror in the unjust, but Justice is synonymous with righteousness, which is the antithesis of Sin.
EDIT: You want Tyrael to be the host because you think it would be ironic, and it would, but it doesn't fit the pattern. That's the thing...there is a pattern. Do you understand what a retcon is, and what it means?
Edited by TDW1996#1743 on 1/5/2013 3:28 AM PST