Who Deserves to be Forgiven?

“…The Lord also hath put away thy sin; though shall not die” Samuel 12:13
How could have God decided who shall live and who shall die? Why didn’t he instead killed David and saved the child? Why did he forgive David’s sin when he didn’t forgive any others?
God, it could be said, is similar to the judge in a court house. The life and future depends on him and his decision, but he is not always fair. He will take into account how the rest of his live was spend and how faithful he was toward him, and he will also de what he thinks will benefit him most. God has many human characteristics, including greed. He wants what’s best for him and this will always affect his decisions. But it is also a mystery why he chooses who will be saved and who won’t because the past of people can influence and how huge the sin was but the fact that they sinned is always a great risk. He sometimes kills some of his beloved but then, luckily, he sometimes saves them, just as he did with David.
Still his sin was not completely ignored since he had to live with the grief of a dying son. He also fated and didn’t sleep during the seven days of his sons life, and was forced to see his wife once more after his betrayal. This punishment was meant for him to never sin again… but will it work? Or is it human nature to sin?