I just had to search for this thread after finding a creepy antartic mission, with a certain Thing. And holy crap was I scared! I figured out immediately they were tough when it took a lot to bring one down, so my 2nd turn instinct was to stun civillians.
Surprise! First civillian was a Thing in disguise, and that right there scared me more than any other mission in my entire time playing X-COM. Needless to say, I had brought only 7 soldiers (although well equipped with .308 and black ops assault rifles and flame gloves, and flares and lights as it was winter in the antartic!) After that nasty surprise, I wasted no time in flame gloving every civilian I saw to death, unsure if they were human or not.
At first I felt a tiny bit betrayed, but given I survived the ordeal with no casualties (thanks to psionics and sheer brutality towards every living thing there) it was a thrill of a mission!
edit: also good work on ufopedia entry
also, civillian death score should possibly be turned off/down/something for that mission? It seemed like the disguised Things counted as dead civilian score, maybe at least when I disintegrated them in one shot