You don't know why Malthael acts as he does. For all we know he disagrees with Tyraels way of "disposing" of Diablo, and acts to kill Diablo by himself. You have no idea what will happen. Way too early to rage.
I saw some liveblog of the conference say that he's trying to end the Eternal Conflict by destroying all demons.
He's out to reap the souls of the demon lords. I guess it fits the title.
He's out to reap the souls of the demon lords. I guess it fits the title.
Hopefully these ideas hold out until release.