User blog comment:A.J AL/Season six plot hole./@comment-2020973-20121228183603

Cass once told Dean it took several angels to get him out of hell and some died, if Cass himself tried to take out Crowley he would have been killed because who knows how many demons there are in hell that would have risen up to fight him. He probbably figured there was nothing in Purgtory that could be a threat to him since it just had monster souls in it.