The stress of being treated as egg-laying machines causes hens to peck at themselves and others, ripping out feathers and eyes, a behavior that is unheard of in chickens raised outside of captivity.
Hens peck at each other to establish primacy in the wild, or in non-factory conditions very like the wild, losing feathers etc. Animal violence is not down to the conditions humans hold them in. Hence the phrase 'pecking order' which originated far before factory farming of chickens. Violence is probably worse in factory conditions and I'm sure some hens will simply go insane given the conditions you describe, but they do naturally peck at each other to establish dominance.
The killing of male chicks and eventual slaughter of “spent” hens are standard industry practice and can only be avoided by rejecting eggs entirely.
They are only standard industry practice in factory farms. On small free range farms or private holdings of chickens, these practices often don't hold. Note that most chickens won't survive in the wild though, so if they are released, they quickly die from predation.
Whether a vegetarian should eat the young of another species harvested before birth is another question (I can see arguments either way), but factory farming is not the only way to harvest eggs and not all eggs are produced that way.
