Compassion is the main ethical principle of one’s relationships with other people and with all living beings, even with non-incarnate ones. This is the main aspect of love on the Earth and the first criterion in the ethical work.
Doing of unnecessary harm to people or to other beings can never be justified in the eyes of God.
But what harm can be considered “necessary”? For example, to cause pain or other damage to criminals when repelling their criminal deeds or defending other people from them. Another example is to punish children that frolic dangerously for themselves and for others. Also — restricting mentally diseased people. And so on.
But revenging oneself cannot be justified: this is an egocentric reaction of the offended lower self that must not be allowed.
The one who realized the true love cannot cause pain to an animal for a meaningless reason. Such a person cannot eat corpses of killed animals: in their dead bodies there is the pain of their death.
For example, Jesus expressed discontent, when He was hinted about the possibility to taste a “sacrificial” lamb at Passover: “Do you think that I am going to eat with you meat at Passover?” (Epiphanius, Haer., 22:4). Neither He nor His disciples ate bodies of animals except for fish; this follows from the words of apostle Peter (Acts 10:10-14).
However, they did kill and eat fish. This is understandable: Jesus did not suggest to people too hard “upheavals” in the stereotypes of their lives. He could not say to fishermen: do not eat fish — fishermen would not listen anymore to such a preacher.
Yet, for modern people it is possible to accept the principle of Love-Compassion as an ethical concept and to follow it as widely and completely as possible within the limits of rationality.
For example, it makes no sense to ponder over whether it is allowed or not to kill a rabid dog or a wolf that attacks man, to kill mosquitoes, gadflies, ticks, etc.
There is also no point in hesitating whether one has a right to kill plants for food, for building a fire, for construction, or to use milk products and eggs for food — we cannot develop on the Earth without doing this. And one’s food must be adequate, with a complete set of indispensable amino acids.
However, killing or maiming plants for no meaningful reason is a different thing: for example, to pick “automatically” a leaf and throw it, to gather a bouquet of flowers, or to buy (to cut) a fir-tree for New Year or Christmas and admire how it dies. This is a meaningless death: people kill these plants not for the reason of survival and development, but out of fancy, because “everybody does this”, or because “I want it!”.
The true compassion originates from awareness that all of us — even vegetal creatures — are God’s children of different age, brothers and sisters of one His family. We all have objectively equal interests in the universe; we all are one.
“Do not let each man look upon his own things, but each man also on the things of others. Let in you be the same disposition as was in Christ Jesus” (Philip 2:4-5).