Oryx had to wait to find Crota's killers to find the dreadnaught and face oryx for the throne of the hive and taken .He also had to wait for the deathsingers to give Crota his last rites on the moon. Also , you dont kill the war priest , the deathsingers, or oryx. the Wp and deathsingers have oversouls and oryx lives in the touch of malice's heart.