Quote from Jinshin
I was thinking earlier about my hypothesis that Malthael wants to do to the Angiris Council what happened to the Prime Evils; have them all absorbed -- perhaps into the Black Soulstone, which can conveniently house Angels as well -- to restore balance. Perhaps that would mean Tyrael himself will be the cause of the End since, having given up his Angelic powers, his Aspect cannot be absorbed into the Black Soulstone to make it whole.Why would Death want to bring Tathamet? he doesn't want to destroy the heavens otherwise Tyrael would be in pieces right now.
But what would happen if it were whole? Anu/Tathamet? Or would it completely cancel out the Prime Evils, stopping their cycle of carnage forever? If so, that could mean Tyrael is involved in the End of Days. Such a grim and ironic tragedy would be a fitting ending, I think, to the Diablo saga.
Quick Edit: Two things I noticed while rewatching the cinematic. One, it looks like Tyrael is hiding the Black Soulstone in a temple beneath a marshland. Sound familiar? Necromancers, anyone?
Also, when Malthael begins to reap Tyrael's soul, he watches it flow into his sickle, then abruptly looks at Tyrael (in surprise?) and casts him away, sparing him. Maybe he WAS trying to absorb his angelic essence and found it useless since Tyrael didn't have any?
About your last sentence, what I think that happened is that the moment Malthael absorbed the first part of Tyrael's soul, Malthael recognized from that moment that he was actually holding Tyrael. Don't forget that Malthael and Tyrael battled as brothers for millenia against evil, so that's why he doesn't want to kill Tyrael (at least not right now). I also think that Malthael was sort of surprised the moment he entered the room for the first time when Tyrael calls his name. The surprise because if you look closely, Malthael moves his head to his right a little bit when Tyrael says "Malthael".