Given the small number of results coming up for the phrase, "no kitty killed" and "no kitten killed" seem like nonce usages by one or a small number of users, rather than an established idiom. It appears to mean that nothing especially bad happened.
In Google, "no kitty killed" only turns up the two results linked in the question. "No kitten killed" turns up one string repeated on five different sites:
The PDF file can be downloaded from here and it is not harmful. No shellcodes, no exploits, no kitten killed. In summary, you can open it with no fear, but do it with a version of Adobe Reader prior to XI ;) (Arsenal).
Compare that to the Server Fault result:
[Under "Pros"] no kitty killed, no risk to crash my desktop by mixing testing/experimental stuff
The two results you link and the usage here seem to suggest a catastrophic or irrevocable event. "No kitty killed" is like saying "Nobody died as a result of running this code or opening this file." It's supposed to be reassuring with a touch of humor. It could be an indirect allusion to Schroedinger's cat, a thought experiment that involves the possible death of a cat.
I found one other result with "no kitten was killed" on StackOverflow:
What would be a better way of doing this? I appreciate you taking the time to answer, but you should know that hearing a dogmatic answer without an alternative solution is also hard, specially when I've done what you say I shouldn't do and no kitten was killed. :)
So that seems to confirm that these phrases are euphemisms for a bad result.