I think regret and guilt go hand in hand. And I believe that they are learned from your experiences. (society, parents, teachers etc.) If you came from a tribe that ate their weakest young - you would not feel regret for eating your kid. If you came from here, and your plane crashed on the top of a mountain, and you ate your weakest young to survive-chances are you would feel regret or guilt as you are doing it. Just another learned behavior.