Istredd studies him for a moment. How deeply do you think the corruption of the Mark had to do with your actions? It isn't to say Lucifer would not have done those things, but it is an interesting question. Could losing the Mark be part of why he was able to be rational, for better or for worse?

All it would do is bring him to the Horizon, which is a place where he can't do any permanent damage, so it isn't a terrible idea. I don't know how people get out of the crater. Meaning they could throw Dean's body into it but how they would get it back out again is a question mark. Supposedly someone could go in too and drag him out? Unknown. What if he was killed and then immediately resuscitated? If his heart stops beating, would the Mark leave him?

This isn't Istredd's problem, obviously, but it is a problem, and getting rid of the Mark will also protect Lucifer from the murderous rage. So he is willing to go into idea mode.

"He's busier than I am. I don't know when the next group of Summoned will get here, I'll deal with it."