You are in the meat birds forum, so we expect to eat them.
You have to make a decision if you are going to eat meat, or not. If yes, then how humane do you wish your protein source to be raised. Most of us here would prefer to know that our food has been raised humanely with lots of space, care, and sunshine vs. disinterested helpers and small cages. We eat meat. We take responsibility for that meat.
Then there are a lot of us who raise chickens for eggs. Meat is not a consideration, only sometimes a by-product of unwanted roosters. For spare roosters, see above.
Then there are those birds that get sick. For those birds, culling (spelled killing) is a humane solution to a suffering animal. You learn to put your "big girl" pants on and do what is best for the animal. Animals don't contemplate their value of life. They only know "I feel healthy, safe and fed" or "I am sick, in danger, or hungry." As soon as an animal is severely sick, their quality of life suffers as they only understand the pain, not a sense of supportive community or mental life. Their flock mates will actually be hazing them, turning on the sick bird creating fear and stress. By the time an animal shows pain, it is usually towards end of life as natural instinct causes the animal to hide its pain to protect if from predators and angry flock mates. So again, that kind of culling is a kindness.
Then there are your aging hens. I simply don't have space to keep all my aging hens, as that soon causes health problems, nor finances to feed those who are not feeding me. I'm in it for the eggs for my family. I try to rotate my layers out by 3 years of age as many people like to get layers but don't want the hassle of chicks. At 3, they are still young enough to lay but are beyond their most productive years. I let the new owners worry about what will happen when the hen grows too old. Most state they are more interested in a pet than food production, so my aging hens go to "retirement" homes for pets with egg benefits.
Those are my thoughts. I'm sure others will chime in too.
LofMc