One way that sin starts

2 Samuel 11:1
1 In the spring, at the time when kings go off to war, David sent Joab out with the king's men and the whole Israelite army. They destroyed the Ammonites and besieged Rabbah. But David remained in Jerusalem.

What happens next in 2 Samuel is David's sin with Bathsheba. I propose that one way that sin starts is contentment. Instead of going out with the army to fight the battles. David stayed home, but this is one of the reasons Israel even wanted a king. They wanted a king that would go out in battle for them. In this case David does not go out. He stays home and just relies on Joab to lead the army. Compare this to the response of Uriah when David calls him back from the battle. David lives in comfort and contentment but Uriah

2 Samuel 11:11
11 Uriah said to David, "The ark and Israel and Judah are staying in tents, and my master Joab and my lord's men are camped in the open fields. How could I go to my house to eat and drink and lie with my wife? As surely as you live, I will not do such a thing!"

(I love the juxtapositions in 1 and 2 Samuel.)