Rechab and Baanah did not want to speak to Ish-bosheth nor did they want any resistance from him before they killed him. Approaching when he was known to take his rest would have allowed them to kill Ish-bosheth quietly without alarming any guard. They had to be quiet if they were to cut off his head since this was likely to take some time. Since they were two of the king’s chief raiders they would have been permitted access to his house without question.1
5 Now the sons of Rimmon the Beerothite, Rechab and Baanah, set out, and about the heat of the day they came to the house of Ish-bosheth as he was taking his noonday rest.