Thank you Gzilla, that pretty much sums it up.
Ultima: first off, Diablo is the main villain. Abaddon is not. If we let your post remain unchanged, then we will have to find a way to give Diablo his superpowers, get rid of Abbadon, and get rid of Diablo. Getting rid of Diablo will not be a priority for the characters while Abaddon is running around, and killing Abaddon first will end the story before Diablo can be sealed in the Core.
"Why would the story end with killing Abaddon? Can't they just continue by killing Diablo next?" In theory, yes, but not in practice. Killing a supervillain will just feel like an ending, and blow all the steam out of the story. And even if that doesn't happen, the same forces that stopped Abaddon can stop Diablo even more easily, resulting in an anticlimactic ending. Either way you look at it, Abaddon has doomed the story.
What were you trying to accomplish by adding Abaddon? Did you just feel the need to give your character more meaning in the story? Was it an attempt to steal the spotlight? Perhaps Abaddon was all part of your plan for somehow establishing Diablo as the villain? Talk to me; why is Abaddon here?
Did you REALLY just kill off the main villain?