"You're the one that said you shouldn't be talking to me about the crime scene, though you were wrong to assume my appearance there was random. In any case, blame The City. Your murderers aren't native, they were brought in. Just like you."
No, he was not going to explain how he deduced that. Sometimes it was fun to see the look on someone's face when he explained and sometimes it felt brilliant to rub someone's nose in it. Sherlock wasn't in the mood for either.
"The victims are native. Survivors won't be. Find yourself a survivor before the boy realizes he should stick to natives, then you'll have yourself a witness, possibly an identity."