Drel & Margo truly had a great friendship in the past. Drel was considered the strongest, but Margo was the most talented. In their match, talent beat strength. It was Margo who was titled the swordmaster and given regalia. Margo was also much bolder and less careful than Drel. It is interesting how Regalia are affected by their first kill. It literally absorbed the dragon. Though the tragic part was Margo was going to get the kill, the smallest mistake cost it, but he was able to hand it off to Drel.
We know the dragon was a servant to Vordin, Lord of the Dark Beasts. So more than likely, Drel is in the same situation as Alicia. Though if this is true, what is Vordin planning with letting Drel do as he pleases? Vordin feels like he has to be plotting something.
Also, to add, what are the secrets that the king is hiding? Now a headless king has to be the work of necromancy of sorts? Another form of magic? Could this be Vordin's doing? Though is the will of the King still there? Potentially throwing his grandson in the forge. Either it is to stop something significant, or are Hiedans themselves the key to the Regalia beyond their knowledge?