All the scriptures of Hinduism focuses on "satvik bhojan" which essentially means vegetarian meals. So why & when did section of society started eating non-veg? Is it allowed by veda's for cetrain section of society like "Kshatriya's"?
Hunting and eating meat among kstatriyas were not a sin. When brahmanas offered sacrifices, the consuption of meat was not a sin. The problem came when brahmanas started to sacrifice animals only for enjoying the taste of the meat. (Thats why -oneof the reasons- Vishnu came in the form of Buddha, to stop this unnecessary animal slaughter) There are some stories in itihasas, when some persons make a vow in which they promise, they wont consume meat (and other foods as well).
Jayadeva Gosvāmī reveals another dimension of Buddha's mission: nindasi yajña-vidher ahaha śruti-jātaṁ sadaya-hṛdaya-darśita-paśu-ghātam keśava dhṛta-buddha-śarīra jaya jagad-īśa hare "My dear Lord, you have assumed now the buddha-śarīra, body, just to, by taking compassion on the poor animals, and therefore you are also deprecating the animal sacrifices recommended in the Vedas.
Satvik Food is an invention of Kaliyuga. Humans by nature is a meat eater.
Shiva and Arjuna fought after a swine. If vegetarianism was in vogue, this chapter wouldnt be there at all.
Sage Agastya digested the indigestible Demon Vatapi. This is not considered as cannibalism because Vatapi always took form a Goat and his brother will invite Rishis for lunch and slaughter Vatapi in goat form and offer it to the rishi. After the lunch was over, the brother will call out - Come out Vatapi.
Vatapi will tear off the belly of the rishi and come out.
Sage Agastya put a stop to it by caressing his belly after the sumptuous lunch. When Vatapi was called to come, Sage smiled and said he is digested. and that was the end of it.
There was no mass slaughter of animals. A group will go to the jungles, hunt animals and share the meat among families.
The prime meat known as "Undercut" was always offered to the Guru. Life went on.
What I state now is my assumption.
Humans are greedy by nature. May be on few occasions the Guru was not offered any meat, forget undercut.
After not getting meat for days together, the Guru might have started boycotting the group and started preaching "Meat is bad for health"!!!
Buddha preached non-violence not vegetarianism. I have Visited any of Buddha Temple in East and Far east, and find devotees offering Cuttle fish, and other meats. Most of the Buddhist eat Beef.
None of the Purana or Upanishads preach vegetarianism.