Science tells us that whatever one eats has an effect on one’s behaviour. Why then does Islam allow Muslims to eat non-vegetarian food, since the eating of animals could make a person violent and ferocious?

Only Eating of Herbivorous Animals Allowed
I agree that what a person eats has an effect on his behaviour. This is one of the reasons why Islam prohibits the eating of carnivorous animals like lion, tiger, leopard, etc., who are violent and ferocious.

The consumption of the meat of such animals would probably make a person violent and ferocious. Islam only allows the eating of herbivorous animals like cow, goat, sheep, etc., that are peaceful and docile. Muslims eat peaceful and docile animals because Muslims are peace-loving and non-violent people.

The Qur’an Says the Prophet Prohibits What is Bad
The Qur’an says:
“The Prophet commands them what is just and prohibits what is evil.”

“He allows them as lawful what is good (and pure) and prohibits them what is bad (and impure).” [Al-Qur’an 7:157]

“So take what the Messenger assigns to you and deny yourselves that which he withholds from you.” [Al-Qur’an 59:7]

For a Muslim, the Prophet’s statement is sufficient to convince him that Allah does not wish humans to eat some kinds of meat while allowing some other kinds.

adith of Muhammad (pbuh) Prohibiting Eating of Carnivorous Animals
According to various authentic Ahadith narrated in Sahih Bukhari and Sahih Muslim, including the hadith narrated by Ibn Abbas in Sahih Muslim, Book of Hunting and Slaughter, Hadith No. 4752, and Sunan Ibn-I-Majah, chapter 13, Hadith No. 3232 to 3234, the Holy Prophet (pbuh) prohibited the eating of:

(i) Wild animals with canine teeth, i.e., meat-eating carnivorous animals. These are animals belonging to the cat families such as lion, tiger, cats, dogs, wolves, hyenas, etc.

(ii) Certain rodents like mice, rats, rabbits with claws, etc.

(iii) Certain reptiles like snakes, alligators, etc.

(iv) Birds of prey with talons or claws, like vultures, eagles, crows, owls, etc.

There is no scientific evidence that proves beyond doubt that eating non-vegetarian food makes one violent.