He finally understood how family feuds started. With simple misunderstandings that involved death. It sounded weird to say, but he could see it was the truth. Your son shoots someone who he thinks is trying to shoot him. Turns out that his brother claims he didn't even own a gun. Now you have a generation to generation feud that lasts for decades.
That same thing almost happened to his team. He considered McGee to be as good as his son, so when he said that the man had pointed a gun at him, Gibbs believed him. Then they found out the man he had shot had been an undercover cop.
Gibbs had been ready to strangle the lieutenant from the metro PD for even suggesting McGee shot without provocation, while the man's partner had been ready to strangle Gibbs for even digesting his partner was a dirty cop. McGee was on a guilt-trip breakdown and Gibbs hated the Metro cops even more for hurting McGee like this.
It was Jenny who smoothed things over. She was the one who ordered them to cooperate with Metro. To keep civil long enough to get to the bottom of things. It was his fiery redhead who'd kept a head on her shoulders and been rational enough to prevent a blood bath. He guessed he shouldn't have been surprised. Most blood feuds took place in patriarchal communities. Here at NCIS, Mamma was in charge.