Cain was cursed for killing his brother Abel out of jealousy, which was the first time that human beings deliberately killed people. Moses killed the Egyptian because he wanted to save his countrymen. Although he was not cursed, Moses actually paid a heavy price, gave up the life of a prince and wandered in the wilderness for forty years.
There is no obvious difference between what God told Adam and what Eve told the snake. Everything is for sure. But the sly snake later said something different to Eve. The snake said that you might not die after eating this fruit, which puzzled Eve.