Human cruelly to animals is unjustified. Just because the practice is endemic it does not justify imprisoning animals within fences, walls or cages, for life. Neither does it vindicate the mistreatment of animals during their incarceration with the imposition of unnatural breeding, feeding and drugging practices, nor the untimely death sentences imposed.
The environment is being damaged by livestock. Currently less than half the world's population, of 7 billion, anticipate meat to be part of their diet, yet this requires the feeding and processing of 70 billion animals a year. This vast number exploits 75% of all available farm land (45% of all land), uses 75% of all antibiotics produced, contaminates the majority of available potable water, whilst emitting over half of all human caused global warming gasses. It is now established by Jeff M. Anhang, Robert Goodland, Richard Oppenlander and many others that a 25% reduction of livestock numbers would instantly return the world's atmosphere back to pre-Kyoto levels, would free up huge areas of land for the re-establishment of biodiversity of flora and fauna, as well as free up an abundance of grain and vegetables to feed every human being.
Meat eating is unhealthy. Studies in longevity all show an average healthy lifespan of vegans to be 10 years longer than meat eaters who are otherwise living in the same socioeconomic environment. The modern addiction to eating animal products is concurrent with the modern demands for pharmaceuticals simply to maintain a life expectancy similar to their ancestors who ate considerably less meat, and took no medications.