Malthael better have a good reason...

The writers better find a good reason behind Malthael's change of heart. Because death in itself isn't evil. Death is a natural event, it is the outcome of life. It defines what life is. It isn't evil. Malthael could be the angel of Death and still be part of the council.

They better not pull off another lame storyline à la Deckard Cain...

While you'll have to wait awhile longer yet to experience the story of Reaper of Souls firsthand, I'd highly recommend checking out Diablo III: Book of Tyrael if you're interested in finding out a bit more about what happened between Diablo III and Reaper of Souls.

As far as Malthael it's not really a change of heart. As before, he wants to see an end to the Eternal Conflict and for angels to be victorious. The thing is, now that the Lords of Hell have been dealt with and the Prime Evil has been defeated, in Malthael's eyes, that leaves one remaining demon faction to be dealt with: humans.

In essence, since humans are descended from the mingling of angels and demons, to Malthael that means that they are actually the largest and most powerful demon faction still in existence. As such, they inherently represent a threat to angels, and Malthael intends to extinguish them and end the Eternal Conflict once and for all.