It was King Aston's dream to become the king of the Barren firmament…a true king acknowledged by the heavens.
So, if he tried to kill Lucas or destroy his barrier right now using Heaven's bane stone, his chances of becoming the true king would almost hit zero.
Moreover, he could do whatever he wanted to do after becoming the true king. At the time, killing others who were bestowed a status by the heavens would completely become legal up to some mark.
So, all he needed to do right now, is control his anger and deal with the Heavenly spirit beast before it stabilizes the heavenly power and becomes the true king of the barren firmament.
'Even though heaven's challenge stone is a priceless item, I have to use it for the current situations; otherwise, there is no use keeping this heaven's challenge stone with me after today.'