Who stabbed Abner in the gateway at Hebron?

Answer: Joab

Joab killed Abner to avenge his brother Asahel, whom Abner had killed in battle at Gibeon. David mourned Abner publicly.

“As soon as Joab had left David, he sent messengers after Abner, who brought him back from the well of Sirah. But David was unaware of it.”— 2 Samuel 3:26 (BSB)