John Gill Commentary 2 Samuel 3:25

"Thou knowest Abner the son of Ner, that he came to deceive thee, and to know thy going out and thy coming in, and to know all that thou doest." — 2 Samuel 3:25 (ASV)

You know Abner the son of Ner
You cannot be ignorant what a cunning deceitful man he is, nor of his designs; or do you not know? are you ignorant? so read the Septuagint, Vulgate Latin, Syriac, and Arabic versions, with an interrogation:

that he came to deceive you :
with false hopes, or to lead into wrong measures with an intention to involve and ruin him:

and to know your going out, and coming in ;
the affairs of his court, the secrets of his government, to observe his conduct and behaviour, and all his actions, and improve them against him:

and to know all that you do ;
he suggests that he came not as a friend, but as a spy, and therefore ought to have been taken up, and detained, and not dismissed. This Joab said to set David against him, fearing, if he should be received into favour, he would be a rival of his; and besides his breast was full of revenge against him for the death of his brother.