My head cannon:
Jergal brought the Dead Three back to life to save Kelemvor.
Some dataminers found that Jergal was serving us because he brought the dead three back to life. Now this has been removed from the story - so head cannon - but I think he did that to save Kelemvor.
Kelemvor was originally Lawful Good but when he started allowing the dead to repent in the afterlife he was but on trial and forced to keep many of the aspects of the portfolio he didn't like: wall of the faithless, fate of the false, deals with devils.
But he kept them and became an impartial judge of the doomed.
Unfortunately Kelemvor is in trouble after the spellplague. The god of murder has lost the portfolio, the god of necromancy is MIA as is the lord of Tyrants. Jergal knows that Kelemvor will be need to take on those portfolios unless someone else takes up the task. And this would change him entirely. He could no longer hate undead, no longer be an impartial judge, no longer wish that the dying pass peacefully. Instead he would be a god of murder, tyranny and necromancy -- just as Jergal was once upon a time.
Jergal doesn't want so see somone he respects be forced to become something he hates so bring the dead three back to life. And that makes AO very angry . . .
