I had thought he was testing if the hero was under any mental manipulation (murder compulsion now that he had a real blade, removing the man the princess considers an enemy, etc); the one thing against my theory is the fact he held the heroes hand the entire time he was being stabbed, which I truly doubt he couldn't physically stop him if the case truly was preventing himself from being killed; he probably did force the hero to stab him. Still could be to test if it unlocks unbridled blood-lust, but we'll see next chapter why.