A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
2 Kings (2 Samuel) 4:4
sa2 4:4BAANAH AND RECHAB SLAY ISH-BOSHETH, AND BRING HIS HEAD TO HEBRON. (Sa2 4:1-2)
Jonathan, Saul's son, had a son that was lame of his feet--This is mentioned as a reason why, according to Oriental notions, he was considered unfit for exercising the duties of sovereignty.
2 Kings (2 Samuel) 4:5
sa2 4:5Rechab and Baanah went and came about the heat of the day to the house of Ish-bosheth, &c.--It is still a custom in the East to allow their soldiers a certain quantity of corn, together with some pay; and these two captains very naturally went to the palace the day before to fetch wheat, in order to distribute it to the soldiers, that it might be sent to the mill at the accustomed hour in the morning.
2 Kings (2 Samuel) 4:7
sa2 4:7when they came into the house, he lay on his bed--Rechab and Baanah came in the heat of the day, when they knew that Ish-bosheth, their master, would be resting on his divan; and as it was necessary, for the reason just given, to have the corn the day before it was needed, their coming at that time, though it might be a little earlier than usual, created no suspicion, and attracted no notice [HARMER].
gat them away through the plain--that is, the valley of the Jordan, through which their way lay from Mahanaim to Hebron.
2 Kings (2 Samuel) 4:8
sa2 4:8They brought the head of Ish-bosheth unto David . . . and said, Behold the head of Ish-bosheth--Such bloody trophies of rebels and conspirators have always been acceptable to princes in the East, and the carriers have been liberally rewarded. Ish-bosheth being a usurper, the two assassins thought they were doing a meritorious service to David by removing the only existing obstacle to the union of the two kingdoms.
2 Kings (2 Samuel) 4:12
sa2 4:12DAVID CAUSES THEM TO BE PUT TO DEATH. (Sa2 4:10-12)
slew them, and cut off their hands and their feet--as the instruments in perpetrating their crime. The exposure of the mutilated remains was intended as not only a punishment of their crime, but also the attestation of David's abhorrence.