Notes on the Bible, by Albert Barnes, [1834], at sacred-texts.com
Beeroth - See the marginal reference. From Jos 9:17, it might have been expected that the population of Beeroth would be Canaanite. But from some unknown cause the Canaanite inhabitants of Beeroth had fled to Gittaim - perhaps the same as Gath - and continued there as sojourners. If this flight of the Beerothites took place at the time of Saul's cruel attack upon the Gibeonites Sa2 21:1-2, Baanah and Reehab may have been native Beerothites, and have been instigated to murder the son of Saul by a desire to avenge the blood of their countrymen. The fact of their being reckoned as Benjamites is quite compatible with their being Canaanites by blood.
This mention of Mephibosheth seems to be inserted here partly to show that with the death of Ish-bosheth the cause of the house of Saul became hopeless, and partly to prepare the way for the subsequent mention of him Sa2 9:1-13; Sa2 16:1-4; Sa2 19:25.
Lay on a bed at noon - Render, "was taking his midday rest," according to the custom of hot countries.
As though they would have fetched wheat - This is a very obscure passage, and the double repetition in Sa2 4:6-7 of the murder of the king and of the escape of the assassin, is hard to account for. Rechab and Baanah came into the house under the pretence of getting grain, probably for the band which they commanded out of the king's storehouse, and so contrived to get access into the king's chamber; or, they found the wheat-carriers (the persons whose business it was to carry in grain for the king's household) just going into the king's house, and by joining them got into the midst of the house unnoticed. If the latter be the sense, the literal translation of the words would be: "And behold (or, and there) there came into the midst of the house the carriers of wheat, and they (i. e. Rechab and Baanah) smote him, etc."
Cut off their hands ... - After they were dead. Their hands and feet were hung up in a place of public resort, both to deter others and also to let all Israel know that David was not privy to the murder of Ish-bosheth.