"If Ancient Devil Emperor killed a demi-god, what would he gain from obtaining a core and its blood? Minus the whole being erased business, mind you." Han Sen wondered this, reflecting on all he had been told in his conversation with Dragon King.
With the magnitude of Ancient Devil Emperor power, he was the sort to have no problem ascending to the Fourth God's Sanctuary and becoming a demi-god. From what he had heard, there would be no need for him to try something as risky as challenging a demi-god in the Third God's Sanctuary.
There must have been a great benefit or bonus to achieve and retrieve, by willingly summoning a demi-god and attempting to kill one. Even the weakest demi-god would be no laughing matter to deal with.
What transpired after Asura killed the demi-god went completely against what anyone imagined might happen. Even Ancient Devil Emperor must not have expected the results, and so that was a catalyst for Han Sen asking this.