Islam says eat the herbivorous animals and not the carnivorous. Also, sea foods are made Halal(allowable). And also,
Prohibited to you are dead animals, blood, the flesh of swine, and that which has been dedicated to other than Allah , and [those animals] killed by strangling or by a violent blow or by a head-long fall or by the goring of horns, and those from which a wild animal has eaten, except what you [are able to] slaughter [before its death], and those which are sacrificed on stone altars, and [prohibited is] that you seek decision through divining arrows. That is grave disobedience. This day those who disbelieve have despaired of [defeating] your religion; so fear them not, but fear Me. This day I have perfected for you your religion and completed My favor upon you and have approved for you Islam as religion. But whoever is forced by severe hunger with no inclination to sin - then indeed, Allah is Forgiving and Merciful. (Qur'an 5:3)
is it immoral to be a vegetarian and not eat killed animals in Islam?
Doctors say that it is highly advisable to have green leafy vegetables. But if a person doesn't like to eat them, is he being immoral?
Similarly, Allah says, that meat has a nutritious value for humans, then if someone Muslim prefers to be a vegetarian for any reason is not being immoral.
Meat is rich in protein, iron, vitamin B1 and niacin. Non-vegetarian food is a good source of excellent protein.
You're curious by thinking how Islam justifies eating animals and if you wish to get you're questions answered, like...
Hindu scriptures give permission to have non-veg
It is not compulsory for a Muslim to have non-vegetarian food
Then this article is especially for you...
Questions Commonly Asked by Non-Muslims - V : Eating non-Vegetarian Food
May the creator guide us all.