Here’s the Buddhist take on it, which I find useful:
Take a look at the root motivation of the action. The “three poisons” are the forces seen as being at the root of all harmful actions: greed, anger/hatred, and self-delusion. If they are what is driving an action, then that action is immoral.
Of these, self-delusion is really the most basic. It can be summed up as the conviction that “I” am fundamentally separate from “you”. It is a perverse blindness to unity. An essential precondition for causing intentional harm to another is seeing them as “other” in the first place.
Greed and anger/hatred are actually just particular manifestations of self-delusion, but they’re such common and harmful faces of the self/other paradigm that they merit their own status as “poisons”.
What the “poisons” poison is our sense of compassion. Compassion is a recognition of our underlying unity, and it is our innate guide to acting morally. But when the poisons hijack our minds, they disable our compassion and divert our energies to self-service. That’s when we start acting harmfully.