Why couldn't Tyrael and the Angels destroy the Black Soulstone?
# 1 Aug 25, 2013
# 21 Sep 17, 2013
RoS' writers address the alleged Black Soulstone plothole:Indeed! This post may offer some additional food for thought on the subject as well.
According to them, since the Black Soulstone isn't coming from the Worldstone like the regular Soulstones, Tyrael doesn't know if it can be destroyed by the Hellforge nor does he want to find out the hard way. So sealing it away seems the best option.