2 Samuel 3 and 4

In verse 26-27 it reads:

Then Joab left David and sent messengers after Abner. They brought him back from the well of Sirah, but David was unaware of it. When Abner returned to Hebron, Joab pulled him aside to the middle of the gateway as if to speak to him privately, and there Joab stabbed him in the stomach. So Abner died in revenge for the death of Ashel, Joab's brother.

So Joab called him back without Davids permission so that is automatically wrong. It later says that David wept, why was he sorrowful he was not the Lords anointed, maybe if we go back a little it says in verses 24-25:

Joab went to the king and said, "what have you done ?" Look here, Abner came to you. Why did you dismiss him? Now he is getting away! You don't know that Abner son of ner came to deceive you and to find out about all your activities and everything that you are doing?

So possibly Joab lied trying to get David to kill Abner when they were friends.