1)
Why does it say "Leshamro v'Lahamiso"?
1.
Radak: To guard him at night lest he flee, and to kill him in the morning.
2)
Why did he not command to kill him immediately?
1.
Radak: He would not kill him at night in front of his daughter; he intended to bring him in the morning, and they will kill him.
2.
Malbim: He sought a justification to kill him - he was Mored b'Malchus for leaving Sha'ul without permission. He sent guards to guard him at night; he would judge him in the morning. If not for the guards, the fact that he chose not to flee would show that he is not rebelling. Also, perhaps David will oppose the guards, and be liable for his own death.
