If you are familiar with the way chickens are killed for consumption in factory farms, you know about many of the horrors they face.
Overcrowded conditions make them peck each other to death out of stress. Females often have their beaks burned off with a hot blade to prevent this, and they subsequently starve; males, considered useless, are often thrown into bags to suffocate.
You also know that there are more humane ways of slaughtering animals. Controlled-atmosphere killing (or CAK), though still killing, is a more humane method to use.
It involves placing an animal into a container that lacks oxygen, but contains argon or other elements to make the animal lose total consciousness. Hens can lose consciousness within 20 seconds, enabling a much less painful death without the suffering.