You killed Morgan Pendleton with steam
When you head down to the steam room to eliminate Morgan Pendleton you will see a room to the left as you head down the stairs. Go into this room and you will find a steam pipe and a Crank Wheel, pick up the wheel and place it on the pipe and turn it. This will pump steam into the room with Morgan Pendleton inside and net you this achievement.
Thanks to Jake2355 for the following tip:
He isn't always in the steam room for everyone. You can take him down with a sleeping dart and relocate his unconscious body into the steam room and use the valve.
Thanks to xblChoas for this video walkthrough