Ferdowsi’s principal source of information and legend was the Middle Persian Pahlavi Zoroastrian work, the Khvatay-Namak (Khodai-Nama). In one of the earlier legends in Ferdowsi’s epic the Shahnameh, Zahhak was the 'foreign' (perhaps Assyrian who Ferdowsi 'Arabized') king who overthrew legendary Aryan King Jamshid. In the story immediately below, Ahriman, the devil incarnate (Iblis, the Islamic word for the devil, is used in place of Ahriman in some Shahnameh translations) brings Zahhak under his control by becoming his cook and seducing him with the taste of meat. Zahhak develops a taste and fondness for meat and comes under the control of Ahriman, a div (evil being). What follow are the overthrow of King Jamsheed by the Zahhak and the completion of the first great Aryan tragic cycle. The Aryan lands which had risen to great glory, now fell on evil times.
As a youth well spoken, clean, and clever,
Ahriman went to Zahhak with fawning words,
"Let me," he planned, "a famed and noted cook,
Find favour with the king with my cooking."
Zahhak was thus by his appetite seduced,
And commanded the monarch's faithful minister
Give to Ahriman the royal kitchen's key.
Foods then were few, yet people did not kill to eat
But lived on the earth's produce of vegetal.
Scheming the evil-doing Ahriman designed
To slaughter animals for food and serve both bird and beast;
That the monarch when possessed
Of the carnal lust for blood and flesh
Would as a slave obey him, and do all his bidding yet.
Prepared he first a meal of yolk,
Whose flavour the monarch relished so
That he praised the wily Ahriman, who replied thus,
"Illustrious monarch! Forever live!
Tomorrow I will serve you and please you well."
The evil one then counselled the king,
That it was blood that gave muscle and strength.
And thus his food would make the monarch lion-fierce.
All night the evil div mused,
What strange repast shall I proffer on the morrow?
And when the azure vault brought back again the golden gem
Ahriman lavishly presented a meal of partridges and silver pheasants.
The Arab monarch gorged
And lost his diminutive wits in admiration.
On the third day Ahriman served lamb and fowl,
And on the fourth a joint of veal with saffron flavour,
With rosewater, musk and old wine.
Zahhak when he had feasted and tasted blood and flesh,
In wonder at his cook's ability, said,
"Worthy friend! Ask now your recompense."
His scheme fulfilled, the Darkness answered,
“Live, O king! In wealth and power.
My heart does throe with your favour my soul's food;
Yet would I ask one boon above my station?
'Tis leave to kiss and lay my face and eyes upon your shoulders."
Surprised Zahhak replied, “I grant it; it may do you grace."
Permission thus received,
Ahriman kissed the monarch’s shoulders and vanished.
A marvel followed – for from the monarch's shoulders
Grew two black snakes.
Distraught Zahhak sought a cure.
Finding none, he excised them,
But they grew back again!
Oh! Strange, like branches from a tree.
The ablest leechers summoned gave advice in turn
And used their curious arts, but all in vain.
Then in leech form Ahriman himself appeared
"This was your destiny," said he.
"Cut not the snakes but let them live.
Give them men's brains and gorge them till they sleep.
Such food may kill them.
It is the only means."
The purpose of the foul div pray shrewdly scan;
Had he conceived perchance a secret plan
To rid the world of all trace of man?
The words above are fairly clear. Before Zahhak's seduction by Ahriman, the devil incarnate, people did not eat meat [see Shahnameh, Zahhak]. In other words, they were vegetarian.
The legend does not end with that answer. When Ferdowsi made the devil, Ahriman the initiator of eating slaughtered animal flesh, he couldn't have made his point about the ethics of eating animal flesh for food more emphatically.
Further, once gripped by the taste of meat and blood, Zahhak became a slave to Ahriman. Perhaps Ferdowsi was trying to say that meat-eating is an addiction to which people become enslaved.
Since the practice of meat-eating requires killing life, it is not hard to see that the practice is placed under the domain of Ahriman, the Lord of Death, the lord of not-life.