Kill Alex Cross - James Patterson

The only way
The President's son and daughter have been kidnapped, and Detective Alex Cross is one of the first on the scene. 

To stop Alex Cross
Just days later, a deadly contagion in the DC water supply threatens  to cripple the capital, and Alex fears that these events could signal the beginning of the most devastating terrorist attack the United States has ever experienced.

Is to kill him
With each hour that passes, the window for solving both crime narrows - and Alex makes a desperate decision that goes against everything he believes in.