At points in the story so far Abner gives flickers of having some degree of character: being loyal to the house of Saul was one - even as Jonathan was loyal to his father (while not begrudging David's favour with God). Also, you wonder if it was respect or fear for Joab why Abner didn't want to kill Joab's brother.
At this stage of the story, however, Abner shows his true diabolical colours:
1. Although he clearly knew that David was chosen by God, it seemed apparent that Joab was David's general, so technically if David became king, Abner would be out of a job - at least at his preferred level.
2. Abner had no fear/regard for God - God was nothing more than a pawn, as was Ishbosheth, in his hands.
3. Abner's ego/ambition was his god
4. Was Abner now trying to move in on Joab's territory? After all his side was losing. His ambition apparently dictated where his loyalties were.
1 1. Be cautious of the favours you accept, you never quite know when and how your benefactor is going to call in the favour.2. Abner represents the Devil in this story - he's in it for number one... himself
3 3. An old man in one of the communities where I grew up would always say, 'A dog, before it bites you, often "grins" at you.' Not everyone who seems friendly is really a friend. Wise up.
4 4. Unbridled ambition is a threat to loyalty. Hence, when forming an alliance, don't just look at what you stand to gain, look also at the value system of your prospective ally.
5 5. Ambition can be blinding as well: while you are scheming to pull one on someone, they may be planning to do the same to you. There's always someone "better at it" than you are. Always better to play it straight.
To read this Bible passage and other blogs visit http://revivedbyhisword.org/en/bible/2sa/3/.