Jamieson, Fausset and Brown on 1 Kings 2:8

8. thou hast with thee Shimei--Though David promised him a pardon, which being enforced by the presence of a thousand followers, could not have been well refused, he warned his son against Shimei as a turbulent and dangerous character. It must not be supposed that in these dying instructions David was evincing a fierce, vindictive spirit. He is rather to be considered as acting in the character of a king and magistrate, in noticing crimes which he had not been in a condition to punish, and pointing out persons of whom Solomon would be under a necessity to rid himself as dangerous to the state. The grateful mention of Barzillai's kindness [1Ki 2:7] was, however, a personal feeling that does honor to the warmth of his heart; and his silence as to Mephibosheth, the son of his beloved Jonathan, would imply the previous death of that prince.