Satan used the victories of war to inflate David's ego and to entice him to rebel against God. The sin of the flesh with Bathsheba was serious and hurt some people but the sin of pride when he counted Israel was far more serious killing 70.000 people. Today we have this great danger within our churches. If satan gets a pastor proud of his preaching or counselling, perhaps a Sunday school teacher proud of his Sunday school, perhaps a church leader proud of the increase of numbers in the church, begins to take control to the restriction of any body else's ideas, or an operator proud of the number of hits the website is getting. Yes we are all vulnerable and we need to be so careful to give all the Glory to God and be a true christian.
Man is a dependant creature.
The essence of sin is to try to be independent from God. It is to take control of our own lives. To try to take God's place in our own life and those of others. If we behave and think independently of God's instruction then satan has a loophole to take over your mind and will. Then we are acting according to the ruler of this world.