Why did they enter again after they already killed him?
Radak: They wanted to take his head 1 to David. They thought that they will find favor in his eyes through this.
Metzudas David: This verse explains how they struck him. First they killed him, and then they cut off his head.
Malbim: Verse 6 discusses when they killed someone else, previously. Now they did similarly to Ish Boshes.
This verse says that they struck and killed him! Perhaps initially they struck him in the fifth rib, from which he would surely die. Now they finished the death and cut off his head. (PF)
Why did no one guard Ish Boshes from them?
Radak (2): They were officers of Ish Boshes. The household thought that they came to talk to him.
Malbim: Ish Boshes did not set up guards, for he did not fear lest someone want to kill him.
