It was during the Vermouth arc I think.
If any of the good guys do killing, it will be pure defensive: To prevent someone else from dying only. And even then there may not be any killing done by the protagonists, because Gosho seems to think that all murder is bad, even when meant to protect someone else: Masumi was made to be wrong for trying to get a hostage taker in a suicide jacket shot for example.
The post I couldn't find about Vermouth because DCTP had an outage was the one where I said she will 1) die sacrificing herself for Silver Bullet and Angel, or 2) she escapes the final confrontation alive and unarrested, but steals an antidote from Haibara so that she could age to her true age and then die as she was meant to because she feels it is the right way.