All living beings are the children of Supreme God. That Supreme God can never be pleased if one harms or kills another creature for fulfilment of one's own desire or taste. A goat or a cow is also the child of that Supreme God and by Killing them one incurs heinous sin. That is the sole reason God has laid down the rule that all human beings should be vegetarian.
Various religious scriptures give this information where God has asked humans to be vegetarian. Overtime due to adulteration & misinterpretation of these scriptures, man has moulded the meanings of these messages from God to his own taste and started following malpractices
29 Then God said, “I give you every seed-bearing plant on the face of the whole earth and every tree that has fruit with seed in it. They will be yours for food.
30 And to all the beasts of the earth and all the birds in the sky and all the creatures that move along the ground—everything that has the breath of life in it—I give every green plant for food.” And it was so.
It is best not to eat meat or drink wine or do anything else that causes problems for other followers of the Lord.
ਜੇ ਰਤੁ ਲਗੈ ਕਪੜੈ ਜਾਮਾ ਹੋਇ ਪਲੀਤੁ ॥
ਜੋ ਰਤੁ ਪੀਵਹਿ ਮਾਣਸਾ ਤਿਨ ਕਿਉ ਨਿਰਮਲੁ ਚੀਤੁ ॥
If one's clothes are stained with blood, the garment becomes polluted. Those who suck the blood of human beings-how can their consciousness be pure?
ਕਬੀਰ ਜੋਰੀ ਕੀਏ ਜੁਲਮੁ ਹੈ ਕਹਤਾ ਨਾਉ ਹਲਾਲੁ ॥
ਦਫਤਿਰ ਲੇਖਾ ਮਾਂਗੀਐ ਤਬ ਹੋਇਗੋ ਕਉਨੁ ਹਵਾਲੁ ॥੧੮੭॥
Forcefully and without any compassion you kill a goat etc. and say that you are doing halal. When your account will be called for in the court of God, then who will save you there?
ਕਬੀਰ ਖੂਬੁ ਖਾਨਾ ਖੀਚਰੀ ਜਾ ਮਿਹ ਅਮ੍ਰਿਤੁ ਲੋਨੁ ॥
ਹੇਰਾ ਰੋਟੀ ਕਾਰਨੇ ਗਲਾ ਕਟਾਵੈ ਕਉਨੁ ॥੧੮੮॥
Superior most food is khichri (dish made from rice and lentils with salt), eat that. One who cuts someone else's neck has to pay back.
ਕਬੀਰ ਭਾਂਗ ਮਾਛੁਲੀ ਸੁਰਾ ਪਾਨਿ ਜੋ ਜੋ ਪ੍ਰਾਨੀ ਖਾਂਹਿ ॥
ਤੀਰਥ ਬਰਤ ਨੇਮ ਕੀਏ ਤੇ ਸਭੈ ਰਸਾਤਲਿ ਜਾਂਹਿ ॥੨੩੩॥
Whosoever takes Marijuana, fish and alcoholic drinks, then despite doing pilgrimages, keeping fasts and following religious practices they still end up in hell.
ਰੋਜਾ ਧਰੈ ਮਨਾਵੈ ਅਲਹੁ ਸੁਆਦਿਤ ਜੀਅ ਸੰਘਾਰੈ ॥
ਆਪਾ ਦੇਖਿ ਅਵਰ ਨਹੀ ਦੇਖੈ ਕਾਹੇ ਕਉ ਝਖ ਮਾਰੈ ॥੧॥
You keep fasts to please Allah (God), while you murder other beings for taste. You only look after your own interests and don't care about others. You are wasting your time, God will never be pleased with you.
ਅਸੰਖ ਮੂਰਖ ਅੰਧ ਘੋਰ ॥ ਅਸੰਖ ਚੋਰ ਹਰਾਮਖੋਰ ॥
ਅਸੰਖ ਅਮਰ ਕਿਰ ਜਾਹਿ ਜੋਰ ॥ ਅਸੰਖ ਗਲਵਢ ਹਤਿਆ ਕਮਾਹਿ ॥
ਅਸੰਖ ਪਾਪੀ ਪਾਪੁ ਕਰਿ ਜਾਹਿ ॥ ਅਸੰਖ ਕੂੜਿਆਰ ਕੂੜੇ ਫਿਰਾਹਿ ॥
ਅਸੰਖ ਮਲੇਛ ਮਲੁ ਭਖਿ ਖਾਹਿ ॥ ਅਸੰਖ ਨਿੰਦਕ ਸਿਰਿ ਕਰਹਿ ਭਾਰਿ ॥
Countless are fools, blinded by ingnorance. Countless are thieves and embezzlers. Countless impose their will by force. Countless cut-throats and are ruthless killers. Countless are sinners who keep on sinning. Countless are liars, wandering lost in their lies. Countless are wretches, eating filth as their food.
Garib Das Ji Maharaj has written various banis in his Granth Sahib which shun meat eating and describe the graveness of this sinful act.