The standard of life for human beings cannot be applied to animals. The tiger does not eat rice and wheat or drink cow's milk, because he has been given food in the shape of animal flesh. Among the many animals and birds, some are vegetarian and others are carnivorous, but none of them transgress the laws of nature, which have been ordained by the will of the Lord. Animals, birds, reptiles and other lower life forms strictly adhere to the laws of nature; therefore there is no question of sin for them, nor are the Vedic instructions meant for them. Human life alone is a life of responsibility.
It is wrong, however, to think that simply by becoming a vegetarian one can avoid transgressing the laws of nature. Vegetables also have life, and while it is nature's law that one living being is meant to feed on another, for human beings the point is to recognize the Supreme Lord. Thus one should not be proud of being a strict vegetarian. Animals do not have developed consciousness by which to recognize the Lord, but a human being is sufficiently intelligent to take lessons from the Vedic literature and thereby know how the laws of nature are working and derive profit out of such knowledge. If a man neglects the instructions of the Vedic literature, his life becomes very risky. A human being is therefore required to recognize the authority of the Supreme Lord and become His devotee. He must offer everything for the Lord's service and partake only of the remnants of food offered to the Lord.
(Sri Isopanishad).

Ahiṁsā means not arresting the progressive life of any living entity. One should not think that since the spirit spark is never killed even after the killing of the body there is no harm in killing animals for sense gratification. People are nowaddicted to eating animals, in spite of having an ample supply of grains, fruits and milk. There is no necessity for animal killing. This injunction is for everyone.
(Bhagavad-Gita-----16:1-3-----purport).

The killing of animals in sacrifice to the demigods, who are expansions of the Supreme Personality of Godhead, is sanctioned for those who eat meat. This is a kind of restriction for meat-eating. In other words, the slaughter of animals is also restricted by certain rules and regulations in the Vedas.
(Srimad Bhagavatam-----5:9:17-----purport).

The word yavana means “meat-eater.” Anyone from a meat-eating community is called a yavana. One who does not strictly observe the Vedic regulative principles is called a mleccha. These words do not refer to any particular man. Even if a person is born in a brāhmaṇa, kṣatriya, vaiśya or śūdra family, he is a mleccha or yavana if he does not strictly follow the regulative principles or if he eats meat.
(Sri Caitanya Caritamrta-----2:18:213-----purport).

Prabhupāda: Kali does not eat meat, but it is the śāstra's injunction that those who are unable to give up meat-eating, they may sacrifice one goat, not cow, one small animal before the goddess Kali, on amāvāsya (new moon) day, night, not day, and they can eat it.
(Morning Walk Conversation).

Another fact disclosed herein is that demons are very much fond of sucking blood. Indeed, all of them are meat-eaters and bloodsuckers.
(Srimad Bhagavatam-----7:2:7-8-----purport).

The desire of the living entity to come into the material world is not very difficult to understand. Although one may be born in a family of Āryans, where there are restrictions against meat-eating, intoxication, gambling and illicit sex, still one may want to enjoy these forbidden things. There is always someone who wants to go to a prostitute for illicit sex, to a hotel to eat meat and drink wine, or to a gambling club to enjoy so-called sports. All these propensities are already within the hearts of the living entities (loke vyavayamisa-madya-seva nitya hi jantor). Some living entities can check the desire to enjoy these abominable activities, but others cannot and consequently fall down to a degraded platform.
(Srimad Bhagavatam-----4:29:4-----purport).

The Kṛṣṇa consciousness movement is teaching this higher knowledge of retiring from materialistic life to return to Godhead, but unfortunately many parents are not very satisfied with this movement. Aside from the parents of our students, many businessmen are also dissatisfied because we teach our students to abandon intoxication, meat-eating, illicit sex and gambling. If the Kṛṣṇa consciousness movement spreads, the so-called businessmen will have to close their slaughterhouses, breweries and cigarette factories. Therefore they are also very much afraid. However, we have no alternative than to teach our disciples to free themselves from materialistic life. We must instruct them in the opposite of material life to save them from the repetition of birth and death.
(Srimad Bhagavatam-----6:5:20-----purport).

The meat-eaters are generally called mlecchas. But all meat-eaters are not mlecchas. Those who accept meat in terms of scriptural injunctions are not mlecchas, but those who accept meat without restriction are called mlecchas. Beef is forbidden in the scriptures, and the bulls and cows are offered special protection by followers of the Vedas. But in this age of Kali, people will exploit the body of the bull and the cow as they like, and thus they will invite sufferings of various types.
(Srimad Bhagavatam-----1:16:20-----purport).

Cows, however, are never meant to be killed or eaten by human beings. In every śāstra, cow killing is vehemently condemned. Indeed, one who kills a cow must suffer for as many years as there are hairs on the body of a cow.
(Srimad Bhagavatam-----6:4:9-----purport).

Therefore there is no difficulty in coming in contact with the Supreme Lord (ko 'ti-prayāsaḥ). On the other hand, going to hell requires great endeavor. If one wants to go to hell by illicit sex, meat-eating, gambling and intoxication, he must acquire so many things. For illicit sex he must arrange for money for brothels, for meat-eating he must arrange for many slaughterhouses, for gambling he must arrange for casinos and hotels, and for intoxication he must open many breweries. Clearly, therefore, if one wants to go to hell he must endeavor very much, but if he wants to return home, back to Godhead, there is no difficult endeavor.
(Srimad Bhagavatam-----7:7:38-----purport).

Those who desire to eat meat may satisfy the demands of their tongues by eating lower animals, but they should never kill cows, who are actually accepted as the mothers of human society because they supply milk. The śāstra especially recommends, kṛṣi-go-rakṣya: the vaiśya section of humanity should arrange for the food of the entire society through agricultural activities and should give full protection to the cows, which are the most useful animals because they supply milk to human society.
(Srimad Bhagavatam-----6:4:9-----purport).

In this age, Kali-yuga, everyone is expert in hunting animals, but most of the people are śūdras, not kṣatriyas. According to Vedic injunctions, however, only kṣatriyas are allowed to hunt, whereas śūdras are allowed to eat flesh after offering goats or other insignificant animals before the deity of goddess Kālī or similar demigods.On the whole, meat-eating is not completely forbidden; a particular class of men is allowed to eat meat according to various circumstances and injunctions. As far as eating beef is concerned, however, it is strictly prohibited to everyone. Thus in Bhagavad-gītā Kṛṣṇa personally speaks of go-rakṣyam, cow protection. Meat-eaters, according to their different positions and the directions of the śāstra, are allowed to eat flesh, but never the flesh of cows. Cows must be given all protection.
(Srimad Bhagavatam-----9:6:7-----purport).

This is the mentality of one who is becoming a pure devotee. When one is elevated to the platform of devotional service by the grace of the Lord and the spiritual master, one first regrets his past sinful activities. This helps one advance in spiritual life. The Viṣṇudūtas had given Ajāmila the chance to become a pure devotee, and the duty of a pure devotee is to regret his past sinful activities in illicit sex, intoxication, meat-eating and gambling. Not only should one give up his past bad habits, but he must always regret his past sinful acts. This is the standard of pure devotion.
(Srimad Bhagavatam-----6:2:27-----purport).