Bloodguilt, Atonement, and Mercy

It is said in 2 Samuel 21: 1-14 that Yhwh sends a three-year famine to his people because of their former King Saul's misdeed against the Gibeonites, their old treaty partner. As we know, Saul died earlier in the battle against the Philistines on Mount Gilboa (1 Sam. 31: 1-6) - YHWH'S punishment for Saul's disobedience to his command (1 Sam. 28: 15-19). Why then does his cause have to be retired, and why in David's reign, and why, this time, with a huge national disaster? Is YHWH so vengeful as to continue exact punishment?