As I see it , Buddhist teaching is not against non-vegetarians nor is it against vegetarians. Vegetarians are not un-Buddhist and vice-versa. There are cases that vegetarians put down other's spiritual practices or judge their progress on the spiritual path based on what they eat. In the Maha-sihanada Sutta, the Buddha listed the unnecessary practices he picked up from other practitioners around the region during that time which includes being quite specific about what they eat. He said " by such conduct, by such practice, by such performance of austerities, I did not attain any superhuman states, any distinction in knowledge and vision worthy of the noble ones." Therefore, he left out all these practices from the formula when teaching disciples and establishing his sangha:
"I thought: 'Suppose I were to practice going altogether without food.' ........."
"I thought: 'Suppose I were to take only a little food at a time, only a handful at a time of bean soup, lentil soup, vetch soup, or pea soup.' So I took only a little food at a time, only a handful at a time of bean soup, lentil soup, vetch soup, or pea soup. ....."
" I did not accept food brought or food specially made or an invitation to a meal; I received nothing from a pot, from a bowl, across a threshold, across a stick, across a pestle, from two eating together, from a pregnant woman, from a woman giving suck, from a woman lying with a man, from where food was advertised to be distributed, from where a dog was waiting, from where flies were buzzing; I accepted no fish or meat, I drank no liquor, wine or fermented brew. I kept to one house, to one morsel; I kept to two houses, to two morsels;... I kept to seven houses, to seven morsels. I lived on one saucerful a day, on two saucerfuls a day... on seven saucerfuls a day; I took food once a day, once every two days... once every seven days, and so on up to once every fortnight; I dwelt pursuing the practice of taking food at stated intervals. ........"Such was my scrupulousness, Sariputta, that I was always mindful in stepping forwards and stepping backwards. I was full of pity even for (the beings in) a drop of water thus: 'Let me not hurt the tiny creatures in the crevices of the ground.' Such was my scrupulousness.
........ "Sariputta, there are certain recluses and brahmans whose doctrine and view is this: 'Purification comes about through food.' They say: 'Let us live on beans'... 'Let us live on sesamum'... 'Let us live on rice,' and they eat rice, they eat rice powder, they drink rice water, and they make various kinds of rice concoctions. Now I recall having eaten a single rice grain a day........."Yet, Sariputta, by such conduct, by such practice, by such performance of austerities, I did not attain any superhuman states, any distinction in knowledge and vision worthy of the noble ones. Why was that? Because I did not attain that noble wisdom which when attained is noble and emancipating and leads the one who practices in accordance with it to the complete destruction of suffering." ( Maha Sihanada Sutta)
What people eat doesn't help or hinder their progress on the path to Enlightenment.
When it comes to the practice of compassion there are many outlets to put compassion into practice, eating vegetarian is just one of these outlets. Some might choose this as an outlet, but then there are numerous other outlets to practice compassion. There are people who are vegetarian due to being raised that way or because it is a rule instead of because they are practicing compassion. For this reason, you do see people eat vegetarian but encourage others to practice animal sacrifice, and no sign of compassion although being a vegetarian. This defeat the purpose of developing compassion and it does not help people develop compassion when it is made a rule or forced on people. If they choose to become vegetarian even when they are not required or criticized by authorities , then their eating vegetarian is more likely to be an expression of compassion. When you forbid people from being non-vegetarian , even if everyone in the country eat only vegetarian it still doesn't mean that these people are compassionate.
I would encourage people to eat less meat but for it to actually be an outlet to practice compassion , it should remain a free choice.