Mercy and righteousness

2 Samuel 4:1-12
Two leaders of raiding bands in Saul's son, they went into the inner part of the house of Ish-Bosheth as if to get some wheat, and they stabbed him in the stomach.
Those two took head of Ish-Bosheth traveled all night to David at Hebron. They thought they can get reward from David. But David killed them for they killed an innocent man in his own house and on his own bed.
David gave an order to his men, and they cur off their hands and feet and hung the bodies by the pool in Hebron, But they took the head of Ish-Bosheth and buried ti in Abner's tomb at Hebron.
Once again David's supporters completely fail to understand his attitude to Saul and the royal family. Ishbosheth is given honourable burial, the two murderers are publicly disgraced.
Why?
Good king respects the king's family, even Saul's treated him worse. David's mercy and love make him more honor and respected by his people.
David clarify the good and bed things, make judgement between right and evil men.
How?
Mercy and righteousness are both important!