What went wrong? We should see at least a flashback of Arthas’ wherabouts in The Maw before the Jailer turned him into a soul shard. For example, his brief resistance against his will. Or make a quest from it showing us this flashback. Something to simply make him back before he’s turned to 35 anima.
Also, I’m not sure, but is it Arthas’ voice in the cinematic? Sounds like Arthas voice mixed with The Lich King’s one.
But yeah, we should see something, as his last lines “I see only darkness before me” indicated that after shattering of the Frostmourne, at least small part of his humanity was back.