Aliens vs. Predator

While playing the Human campaign, during the 2nd mission Refinery; you will eventually find yourself in the middle of an alien nest. Follow your objective marker to come face-to-face with Van Zandt. After a brief conversation with him, he will ask you to kill him, so he no longer has to suffer, oblige him with a few bullets and the achievement is yours. Note that you must do this quickly, before an alien bursts from his chest and that as soon as you do this, you will be attacked by 4 or 5 aliens, so be on the ready with your trusty machine gun ;)

When your playing as a marine in the refinery level, you'll come to a part where tequilla says "holy crap, you found the nest" or omething along those lines. There's Van Zandt tethered to a pillar. I set him on fire and got the achievment. Hope this helps!