Agnar had impressive regeneration, but the wounds inflicted upon him by Poppy never stopped bleeding, which was probably due to her deadly poison. His bones worked hard to keep making more blood at a devilish speed, but they were broken and battered, resulting in the kid slowly dying.
His regeneration alone wasn't going to save his life; it was only going to delay his death. But for a devil, that wasn't the end. He could transform into his Rakshasa form and recover, and even if he were to die, he would just resurrect in hell.
