Conceptually, Buddhist ethics is very simple and elegant, at least in my limited understanding.
The state of Buddha (Nirvana) is taken as the gold standard of all the best.
While Samsara, and within Samsara the hell realm particularly, with its absolute
dukkha, is taken as the worst possible condition. Psychologically, subjectively, Hell is defined as state "everything is wrong" -- and Nirvana as "everything is right"1
The good and bad then are defined in relation to these two extremes. Whatever is conducive to psychological suchness is considered good (kusala). And whatever is conducive to psychological wrongness, to staying in or getting deeper into Samsara, is considered bad (akusala).
So in Buddhism the connotation of kusala/akusala is not a moral quality but the effect in terms of propelling us one way or another. Researchers of pre-Buddhist sources suggest that kusala originally meant "wise", "clever", "intelligent", "expert" -- and later "blameless". As in, if you act wisely, you won't be blamed. My favorite choice of translation, is one that Thanissaro Bhikkhu settled on: "skilfull". On one occasion I translated akusala as "pathological" -- kinda the opposite of "healthy", which is another connotation of kusala. Although in informal speech I simply say good (as in "drinking milk is good for you").
To add more depth and color, both long term and short term effect must be considered. While achieving harmony of short term and long term is said to be a sign of a high skill (e.g. enjoying pleasant being in the here and now, and at the same time making progress towards Nirvana), sometimes the two cannot be reconciled at which point long term becomes more important than short term. All this follows from Buddha's explanations in Pali Canon.
Also, both personal as well as global result must be considered. Again, harmonizing the two is a sign of class, but in case of conflict -- Mahayana folks close your ears -- personal result wins. Buddha is very explicit about this. It does not mean it is Ok to achieve personal bliss at the expense of the world, vampire style. It just means, if oxygen masks jump out, first put on yours, and then help your neighbor. This is my understanding of course, but I swear I did not make this up.
There is one distinction that Buddha usually drew and that is between the good that leads to worldly happiness vs. good that leads to Liberation. The two match pretty closely in most basic scenarios (like adhering to five precepts in day to day behavior, which both helps social harmony AND involves taming one's mind), but may diverge on more advanced stages:
... Now, I say that right view is twofold. There is right view that has
cankers, that is on the side of merit, that ripens unto cleaving to
new birth. There is right view that is ariyan, cankerless,
super-mundane, a component of the Way...
This is how it basically is... What else... To answer your question directly, if there is anything like "sin" in Buddhism, it would only be so because of its correlation with bad effect, getting one further from Nirvana and closer to
dukkha. Buddhism is a very practical framework, and because it grounds itself in first-hand experience it's kinda hard to to argue with.
For a more professional take on the topic, check out this piece by Bhikkhu Thich Nhat-Tu. There is also tons of research on this topic in buddhological academia, but I would be very careful with putting my trust in research that does not come from a practicing Buddhist.
To conclude on a practical note, in case you have doubts as to whether it is in our power to develop kusala and abandon akusala here is some encouragement from The Man Himself:
"Abandon what is unskillful (akusala), monks. It is possible to
abandon what is unskillful. If it were not possible to abandon what is
unskillful, I would not say to you, 'Abandon what is unskillful.' But
because it is possible to abandon what is unskillful, I say to you,
'Abandon what is unskillful.' If this abandoning of what is unskillful
were conducive to harm and pain, I would not say to you, 'Abandon what
is unskillful.' But because this abandoning of what is unskillful is
conducive to benefit and pleasure, I say to you, 'Abandon what is
"Develop what is skillful (kusala), monks. It is possible to develop
what is skillful. If it were not possible to develop what is skillful,
I would not say to you, 'Develop what is skillful.' But because it is
possible to develop what is skillful, I say to you, 'Develop what is
skillful.' If this development of what is skillful were conducive to
harm and pain, I would not say to you, 'Develop what is skillful.' But
because this development of what is skillful is conducive to benefit
and pleasure, I say to you, 'Develop what is skillful.'" -- Kusala
Sutta AN 2.19
1not really, as "everything is right" would be the god's realm. Nirvana achieves "everything is right" in a funny way, by transcending the axis of psychologically agreeable/disagreeable, which makes everything right even when it's wrong, but that goes beyond our present discussion. For practical purposes let's pretend Nirvana is a simple "everything is right".