Diablo II's main theme is the corruption of Diablo I's hero. The Warrior carries Diablo, The Rogue is revealed to be Bloodraven, and The Sorcerer, the summoner. (Though doesn't look like him) To add insult to injury Andariel didn't turn the Sisterhood into the same formidable opponents as the Corrupted Knights of D1, they somehow managed to be 1's Redshirts and Mooks at the same time. The Zakarum, which is supposed to be the most powerful institution of the light, has become through D1 and D2 (via Diablo and Mephisto respectively) become the Prime evil's most useful tool.
Like the normal Worf effect, where the bad guy demonstrates their badassedness by handing the "though guy"s rear end back to them (In Star Trek the next generation it's Worf), The Diabloverse Worf effect, demonstrates the power of Diablo's evil by demonstrating Diablo's ability to corrupt not only the toughest, but the "Heroes" and institutions most resistant to corruption. And this begs the question how Incorruptible were the ones corrupted in the first place?
No wonder Cain and Tyrael seem to be next on the menu. That goes for all of d2's heroes as well.