Albert Barnes Commentary 2 Samuel 16:8

"Jehovah hath returned upon thee all the blood of the house of Saul, in whose stead thou hast reigned; and Jehovah hath delivered the kingdom into the hand of Absalom thy son; and, behold, thou art [taken] in thine own mischief, because thou art a man of blood." — 2 Samuel 16:8 (ASV)

All the blood of the house of Saul - Shimei probably held David accountable for a series of deaths: Saul, Jonathan, Abinadab, and Melchishua, who were killed in battle by the Philistines with whom David was allied; Ish-bosheth, who was killed as a result of David’s pact with Abner; Abner himself, whose death Shimei attributed to David’s secret orders; and all 360 men killed in the battle between Joab and Abner (2 Samuel 2:31).

Some also think that the execution of seven men from Saul’s immediate family (2 Samuel 21:8) had already occurred before David’s flight, and that Shimei was referring to this as well.

Shimei’s hatred and virulence indicate that the Benjamites resented the loss of the kingship from their tribe, even at the height of David’s monarchy.