Malbim: If Avner truly rebels against his king, and came without Ish Boshes' permission, he would be afraid to his house, for surely Ish Boshes would kill him when he hears that he made a Bris with David. He should have stayed here to be shielded in your protection from his king! Since he returned, this shows that he came with his king's desire, as a spy. If so, you should not have sent him!