I've almost completed Chapter 3 "Unforeseen Consequences" and haven't seen anything which looks like it could be related to this achievement. What do I need to do in order to unlock this achievement?
The Ethically Questionable achievement comprises of the following actions and occur during the 'Questionably Ethical' chapter;
- Killing an alien grunt with poisonous gas
- Using the tesla discharge on the bullsquid
- Using poisonous gas on the headcrabs in their habitat
- Shooting the headcrabs with a laser
- Firing the snark cannon until it breaks
As mentioned by PWhite these are all segmented off in their own separate rooms where you're generally presented with a button to press to activate lasers.
I am unsure about in BMS, but in the original Half-Life, some room contained what could be considered "questionable experiments." They occur later in the game, and take the form of rooms where there are (usually caged) aliens and a button. Pressing the button will kill the aliens. This may be what you are searching for.
In black mesa firing the snark cannon until it breaks is a separate achievement, you need to shoot the little attack crawler aliens out of the potato cannon thing to finish the ethnically questionable achievement. The room for this is between the grunt that you poison and the bull squid that you zap. I missed it somehow on my first go.