That sucked.
I'm sorry but I find it really hard to believe that angels who know that they may have to tangle with leviathans don't have a plan in case they run into one. Like oh I don't know. One of the angels grabs the leviathan and transports it to the moon.
That ending sucked.
I get Castiel being broken and I get that we discover why he needed to be returned due to his blood being needed but I do not buy at all that angels can be killed by leviathans.
The show already said a long time ago that angels can only be killed with those swords so changing that now is just crap. Totally ruined this episode for me.
Maybe this explains why the angels have not be helping to kill the leviathans but the fact that they don't usually interfere and also they just had a giant war in heaven and a lot of angels died could have been enough of an explanation.
and if you wanted the leviathan to send the angels away he could have used the same trick Dean used. End of story instead of changing your own canon.
And I'm supposed to believe that he second angel just stands there while the bad guy kills his fellow angel and does NOTHING. Seriously show do you really think the viewers are morons?