Then the LORD said to Cain, “Where is your brother Abel?”
“I don’t know,” he replied. “Am I my brother’s keeper?”
The LORD said, “What have you done? Listen! Your brother’s blood cries out to me from the ground. Now you are under a curse and driven from the ground, which opened its mouth to receive your brother’s blood from your hand. When you work the ground, it will no longer yield its crops for you. You will be a restless wanderer on the earth.”
Cain said to the LORD, “My punishment is more than I can bear. Today you are driving me from the land, and I will be hidden from your presence; I will be a restless wanderer on the earth, and whoever finds me will kill me.”
But the LORD said to him, “Not so; if anyone kills Cain, he will suffer vengeance seven times over.” Then the LORD put a mark on Cain so that no one who found him would kill him. So Cain went out from the LORD’s presence and lived in the land of Nod, east of Eden.


Why did you despise the word of the LORD by doing what is evil in his eyes? You struck down Uriah the Hittite with the sword and took his wife to be your own. You killed him with the sword of the Ammonites. Now, therefore, the sword will never depart from your house, because you despised me and took the wife of Uriah the Hittite to be your own.’
“This is what the LORD says: ‘Out of your own household I am going to bring calamity upon you. Before your very eyes I will take your wives and give them to one who is close to you, and he will lie with your wives in broad daylight. You did it in secret, but I will do this thing in broad daylight before all Israel.’”
Then David said to Nathan, “I have sinned against the LORD.”
Nathan replied, “The LORD has taken away your sin. You are not going to die. But because by doing this you have made the enemies of the LORD show utter contempt, the son born to you will die.”

Both of the men killed somebody, God punished both of them, but they both respond to the punishment quite differently. Cain says, This punishment is more than I can bear, while David says, I have sinned against God. I really don't think Abel was sorry for what he did at all. But David obviously was. There's a big defference between being sorry for your sin, and being sorry about the punishment.