Arguably, he already killed millions if not billions of people before getting the stones. Gamora’s home planet for example. This is no Would you kill baby Hitler? conundrum.
Not just that but he and his army were literally in the process of trying to kill the heroes and destroy the Earth (and he does the exact same snap as the previous Thanos, only unsuccessfully because he didn’t realize there were no stones).
That said, considering you can do apparently anything with all the stones, it is odd that Tony chose to the same sort of snap decimation that Thanos did.