For me it would either be in TSWLM when he drops the bald guy off the roof, or when she shoots Stromberg, in the groin it may look like, and then fires several more shots to finish him off.
Or the scene in FYEO, where he kicks the car off the edge of the cliff. What do you think?
To me a cold kill would mean "cold hearted", which would imply something like the most innocent or sympathetic target.
As such, someone like the OP door-opening Russian soldier would be it, for me. Not really in on the scheme, likely just a local boy trying to make his living and get home at the end of the assignment, no awareness of any smuggling or terrorism, in fact more like a cop (for the "wrong" side) who is more-or-less trying to tamp down criminal suspicious activity...just in this case the we want/need the activity to proceed because it is James', in our interest.
I would actually really like if Bond stories tackled the "cold hearted kill" vastly more straightforwardly than they've ever done (though they touch on it better in Fleming's books I think; its been a while so I'd have to reread). The movies are almost always carefully constructed so James never really has to make truly cold/sympathetic kills, which is actually core meat-and-potatoes of the job but not the fantasy.
Even when they play in that area, its still safeguarded against loss of audience support, eg Elektra or Mr. White (cold hearted suicide assistance?). Anyways I think there is interestingly untapped drama there that they avoid.
One thingy I liked about Rogue One (not a nonsequitur) was the truly cold kill early on by Luna's character. Would like more stuff like that in Bond.
Now, this is a signature gun, and that is an optical palm reader.