Every animal used for their flesh died because of the demand by non vegans for meat. Farmed animals do not have happy lives. From the day they are born they endure the pain of mutilation such as dehorning, debeaking, removal of tails, teeth and testicles, castration, and branding, all of which are conducted without anaesthetic or pain relief (not that alleviation of pain would justify such gross bodily assault). Even if they had the happiest possible existence, there is no justification for killing them. All of them are killed years before the end of their natural lifespan. Chickens are a mere six to to twelve weeks when slaughtered for ‘chicken’ flesh. They may look like adults but at the slaughterhouse they still chirp like the babies they are.
Even if they did not endure the suffering that is standard practice in animal agriculture, they all face a frightening and savage death at the slaughterhouse. The fact that a significant percentage of them are conscious during dismemberment is only one of the many reasons to stop harming them by going vegan. The most important reason is that their bodies are not our property. They are not food. None of them wants to die.