Question

Why can we eat ’living’ plants but not ’living’ animals?

Answer

I am going to give the answer from a different perspective. The conception of Guna (property).

There are Three types of Guna

  1. Sattva (the quality of goodness): Sattva is Calm, Saintly. Its brings up quality that allows spiritual holiness, forgiveness and submission to God. Foods dear to those in the mode of goodness increase the duration of life, purify one’s existence and give strength, health, happiness and satisfaction. Such foods are juicy, fatty, wholesome, and pleasing to the heart. (BG 17.8)

  2. Raja (the quality of passion): Foods that are too bitter, too sour, salty, hot, pungent, dry and burning are dear to those in the mode of passion. Such foods cause distress, misery and disease. (BG 17.9)

  3. Tamas (the quality of ignorance): Food prepared more than three hours before being eaten, food that is tasteless, decomposed and putrid, and food consisting of remnants and untouchable things is dear to those in the mode of darkness.Onions and garlic fall into this category.(BG 17.10)

We are what we eat, so you see. When you see the whole picture from the Gunas' perspective it will start making sense altogether. Why we can't eat certain types of foods, why we do not use some types of food in our Pujas (rituals or worship).

In order to be a saint/God's servant, humans must suppress raja and tamas guna, and eating sattvic food is a large part of that.

EDIT:

In Bhagavad Gita, Krishna states what should be used to worship him with Patram (leafs), Puspam (flowers), Falam (fruits), Toyom (water). So, god didn't say worship me with meat. The priest meant to say the same. Krishna only takes Sattvic foods from us. See the Bhagavad Gita 9.26 quote here.

Is it true that if we eat vegetables, we are still taking lives, therefore we're guilty?

Unfortunately, yes. The only way to mitigate this is to offer the food first for sacrifice. This is supported in Bhagavad Gita 3.13:

The devotees of the Lord are released from all kinds of sins because they eat food which is offered first for sacrifice. Others, who prepare food for personal sense enjoyment, verily eat only sin.

There is also a difference in picking fruits or greens from a plant, which does not always kill the plant. Regardless, the food must be offered to get free from negative karma.

There are five types of people who gets equal share of sin of killing an animal.

  1. Person who kills it.
  2. Person who sells the meat.
  3. Person who carries it to home
  4. Person who cooks it.
  5. Person who eats.

Edit

Request to anyone edit this answer If you want to relate science with religion please post your own answer. The answer given is strictly theoretical (how concepts is described in scriptures).

You can also read Chaandhogya Upanishad They says that Your Food divides into 3 parts:

  1. Shthul change into solid waste food. Example-Toilet
  2. Aadhu change into liquid food like blood
  3. Shushm change into mind balancing elements like Brain

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