Firstly, Yahweh did not condemn Saul to death, although I notice in your details that you qualify that statement by saying Yahweh “effectively” condemned Saul to death. Still, I am not sure how you came to this conclusion: Saul remained on the throne as King of Israel for the next thirty-six years or so. In battle with the Philistines, Saul was severely wounded and fell on his own sword – he committed suicide so that he would not be captured and tortured by the Philistines. This can hardly be construed as God “effectively” condemning him to death.
As for comparing the sins of Saul and David, that is not how justice or punishment is administered. Different sins and even same sins are handled differently because there are different circumstances, heart conditions and attitudes involved. Thus Yahweh tells us, at Jeremiah 30:11, that “I will have to punish you to the proper degree.” Manifestly, then, many things are taken into consideration when God administers punishment for a sin or the practice of sin.
David’s sins did not leave him without punishment. He suffered terrible atrocities in his family for the remainder of his life. There is no room to say that God was “more lenient” with David. God personally handled the case and David and Bathsheba did not suffer the death penalty, that is true. You say why did God kill only one person? I assume you are referring to the adulterine child. If God had allowed the case to be judged by the human judges and had the death penalty gone forward, the adulterine child would have died anyway – in Bathsheba’s womb.
What made Saul different from David? His attitude, his heart condition and his practice of disobeying God. Saul, who was shy and humble at the day of his anointing as king, became proud, rash and presumptuous early on. See, for example, 1 Samuel 13:1-14, 1 Samuel 14:24-27, 1 Samuel 15:8-29. It came about that Jehovah God withdrew from Saul and so did His prophet Samuel. Saul was left to his own devices although he continued to reign. If you continue to read about him you will see that he went from bad to worse, finally getting caught up in spiritism and attempted murder in connection with David, even trying to murder his own son Jonathan.
David, on the other hand, was a man agreeable to God’s heart. It is true that David sinned, sometimes badly. But David always repented, always had the right heart attitude, always made sure to not repeat the same mistakes again and always was a force for true worship. This cannot be said of Saul. As for the differences in what befell them, you should consider the life of King Manasseh. 2 Kings 21:2-9 reports on the reprehensible acts committed by this man who ended up being taken captive to Babylon by the King of Assyria. He repented from the heart and Yahweh restored him to his kingship. Yet he did things more reprehensible than Saul and David combined.
It is not wise to compare the sins of individuals with a view to judging the One who judges righteously.
Hannah J Paul