I think it'd be neat if God-Jack shared Death's logic here:
DEATH: Even if I remove Dean from the playing field, we're still left with you, loyal, dogged Sam, who I suspect will never rest until he sets his brother free. 10x23
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GOD JACK: Cas needed to remain locked away, because The Empty can't come to Heaven, not again. We wouldn't survive it.
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GOD JACK: So you see, Dean. I had to kill you. It had to be done. But I... couldn't do it myself. It... I just couldn't.
GOD JACK: So I put out the hit; any monster who could get it done gets entry to Heaven.
GOD JACK: And it worked. (JENNY: [No, I'm not the big boss.] I just called dibs.)
GOD JACK: After that, it was easy. Tell Bobby that Cas helped, let him give that some weight, some truth. You had to think Cas was saved. So you wouldn't meddle.
DEAN: Meddle in what?
GOD JACK: The greater good, Dean. We're at war.