Where does it say that "thou shalt not kill" does not apply to gentiles, in any Jewish religious text from any time? And by that of course I mean "does not apply to gentiles, period, kill them as you'd kill animals, no biggie" as opposed to laws about waging war etc. - which is the only non-weasely interpretation of "does not apply to gentiles"?
You should checkout the old testament. There is god sanctioned gentiles killing aplenty. See the story of Amalek for example
"From man unto woman, from infant unto suckling, from ox unto sheep, so that the name of Amalek not be mentioned even with reference to an animal by saying "This animal belonged to Amalek".
A contemporary Jewish reader would have been amazed at an idea that "though shall not kill" for example, applies to gentiles.