Just weighing in on the question of “sin” in Buddhism:
The moral code in Buddhism is organized around the principle of not causing suffering. There are rough guidelines, in the form of precepts, that are intended to warn us away from activities (like killing, lying, stealing, etc.) that are more likely than not to cause suffering.
But the Buddhist precepts aren’t absolute. There are, though rarely, circumstances where adhering to the letter of a particular precept would increase suffering rather than prevent it. The ultimate moral mandate is not the precept, but one’s own compassion. If the precepts prescribe one course of action (or non-action), but compassion urges another, compassion trumps precept.
Morality in the real world is messy business. We’re limited in our capacity to understand the full consequences of our actions, so we sometimes cause suffering while meaning well. Then too, sometimes there are no “good” options, only lesser or greater suffering. So moral living in Buddhism is seen as a skill that one refines over time. As with all learning, there’s a lot of trial and error involved. A failure of compassion or other misstep along the way is better characterized as an “unskillful action” than a “sin”.
An unskillful action that causes suffering has consequences that ripple out into the world in unpredictable ways, propagating the suffering. That’s what karma is. There is no cosmic arbitrator of all this, dealing out judgments and punishments. It’s a simple matter of cause and effect.
The Three Poisons (Greed, Anger/Hate and Delusion/Ignorance) aren’t offenses in themselves; they’re the underlying pathology behind unskillful actions. They represent fundamental misperceptions about the nature of reality—a failure to clearly see how things are—and so they undermine (or poison) our ability to act skillfully.