The key had unquestionably leaked, and though it wasn't wide spread at the time, it inevitably would be. Things were already starting
Wikileaks said their decision to publish was to prevent third parties from tampering with the leaks creating false stories, but it was likely primarily that Assange and Wikileaks wanted the credit for the leak. Not a noble reason, but it still wasn't their fault they were in that shitty situation.
"Not a noble reason, but it still wasn't their fault they were in that shitty situation."
Not so sure about that. I recall some of the journalists working with him on the release said, they were shocked to here, that Assange said he does not care at all about the life of the informants, as they were working for the US. (source, some article from "Spiegel", would be quite some work to dig that up)
So I do not trust, that he seriously was concerned about their lifes, making serious security considerations.
There was disagreement about the decision, but again the leaks were already out there. As you mentioned, Wikileaks published them on the second. Cryptome published them on the first.
Every possible decision after the keys were leaked was shitty. Maybe Wikileaks could have picked a less shitty one, but they were still in a terrible situation because of somebody else's actions.
My point is, if he would have been concerned, he could have used better security in the first place.
"In February 2011 David Leigh of The Guardian published the encryption passphrase in a book;[6] he had received it from Assange so he could access a copy of the Cablegate file, and believed the passphrase was a temporary one, unique to that file"
Assuming David Leigh was not lying, Assange should have been more clear with the security implications. (then again, I see no reason to publish the temporary key in the first place).
Still at that time it was not not known, except for maybe some intelligence organisations. So if really concerned, one could have done many different things to protect informants, delay the time, instead of publishing it officially for the whole world to see.
I won't claim Assange had great security, I don't think even he would. Still, publishing any key you get without express permission seems suspect.
The key was public and the database was public. If you're an informant, would you rather be completely unaware of that while the local intelligence organization is already digging through it or have the whole world know including people that could help/warn you? I don't think "sit on it" is obviously the best choice.
Wikileaks said their decision to publish was to prevent third parties from tampering with the leaks creating false stories, but it was likely primarily that Assange and Wikileaks wanted the credit for the leak. Not a noble reason, but it still wasn't their fault they were in that shitty situation.