Second book of Samuel verses 1-12

In these verses, David is informed of the death of Saul and Jhonathan, and as it is shown on verse 12, he mourns for both Johnathan and Saul, who was so many times evil to him. Through this, i can derive that there is a contradiction. For, back in Exodus, it is said that there is to be an eye for an eye and a tooth for a tooth, meaning that if something is done to you, you should do exactly the same to he who has done that to you. But here, David is mourning, and fasting for Saul, after he so poorly treated David.