Bhagavad Gita places human beings primarily into three categories viz. Sattvic, Rajasic and Tamasic. Those falling under sattvic category were pure both in deeds and thoughts. Sattvic people always relished vegetarian food and that too in minimum quantity as required to maintain the body.
Rajasic people enjoyed feasting on food both vegetarian and non-vegetarian. Rajasic people are primarily represented as owners, entrepreneurs and kingly people that deployed manpower under them. Rajasic people had no dearth of wealth. They enjoyed a lavish lifestyle.
The third category Tamasic, were those who lived only for the sake of worldly pleasures. For this category of people morality and ethics meant little. For the sake of enjoyment they relished killing innocent animals and eating them. Most Tamasic people were non-vegetarians. Tamasic pravatti represents lethargy, dullness of character.
As ordained by God the spiritual journey for sattvic people was faster compared to the other two categories. Rajasic people may or may not be inclined to indulge in spirituality.
People with Tamasic tendencies rarely indulged in spirituality. Consuming non-vegetarian food does not result in invocation of pure thoughts, necessary for humans to gain spiritual progress.
Spirituality as detailed in Bhagavad Gita demands highest level of sattvic indulgence. Those with Rajasic or Tamasic pravattis failed to indulge in spirituality satisfactorily. But exceptions always existed in the society.
Those with Tamasic pravattis cannot be branded as criminals. They were just delaying their spiritual journey, nothing more.
Any human being no matter what eating practice, one can indulge in Bhagavad Gita any given moment of life. As scriptural texts are better understood being a vegetarian, it is preferable that to indulge in Bhagavad Gita satisfactorily we changed our habit from non-vegetarian to vegetarian.
Spirituality directly relates to our thinking prowess. Those who consumed meat could never invoke positive thoughts most of time. Thoughts of animals getting killed for the sake of their food always prevailed in their thinking process.
As a soul atman to reach stage of absolute purity, invocation of pure thoughts all the time is necessary. This can be done maintaining a Sattvic attitude in life.