The last thing I killed was a vortigaunt in Half-Life: Opposing Force. I've always wanted my own vortigaunt for a friend, but a whole army is even better. No world conquering for me, though, because as I recall, the Nihilanth also tried attacking the Earth with an army of vorts and it didn't end so well for him. So I guess I'll be using my new army to defend the world instead.