Dude! that's what I did, Existence of the karma can be described by the existence and non-existance of the feeling 'regret'.
'regret' is closely connected to the conscience. You can't escape from your own conscience nor 'karma', sound equal?
for psychopaths and animals,
- do they have a conscience? Yes,
- do they sense their conscience? No.
- Do they feel 'regret'? No.
So, they are basically animals,
A lion doesn't regret killing another animal for food. so, after it's death, it will continue to be born as an animal.
But, you see, sometimes, some animals (e.g.dogs) feel 'regret', so they will have the chance to come up or go down...
I disagree with your premise. AFAIK Buddhism doesn't give priority to the feeling "regret" and discriminate other feelings in explaining how karma works. Rather regret is taught as another obstacle, not a virtue, in Buddhism.
Besides it's not "regret" that explains how karma works. It's the exact opposite according to Buddhism.
see this graph? All the cittas are divided into 2 classes namely kusala and akusala. Regret is a combination of some of those akusala cittas and it's mixed with desire and anger. So there are many cittas involved in bringing about the existence of the feeling we call "Regret".
So it's karma that explains how and why beings regret. Not the other way around.



