My read of it was Uther feeling sorry for becoming a vigilante in his own view. (Effectively, WoW Alliance uses Anglo-Saxon legal theory. Have to leave judgement to the Authority for it.)
But re-invoking casting Arthas into the Maw, after it had a cutscene, suggests they intend to torment Uther with it somehow serving the Jailer in the end. (“Devos led me down a dark path.”)
Their pushing Arthas’ soul going to the Maw is part of why I think it’s in Kingsmourne.