He could have stopped it.
That one little bullet. He could have easily have stopped it, could have grabbed the gun when the Doctor had prised it out of Francine's hands and thrown it across the room.
He could have easily have gotten it while it lay on the floor, while everyone ignored it, focusing on the Doctor, as he decided to keep the Master with him, thinking that he finally had someone to take care of.
Jack was the one who noticed Lucy Saxon picking up the gun, lifting it slowly with one hand, and pointing it straight at the Master.
He could have stopped it.
He could have grabbed her hands, forcing the gun away, stopping what he knew was about to happen. He could have even gotten in the way of the bullet, safe in the knowledge that he could survive it, like every other time he had lived through death.
But he didn't.
He just stood there and watched, as the bullet tore through the air, and hit it's intended target. Stumbling backwards, the Master fell, but the Doctor was there, catching him as he fell, and holding him close, softly comforting the other Time Lord.
Jack, trying to ignore the affection the Doctor was giving to the man who had killed millions, slowly moved towards Lucy, prising the gun out of her hand. Why he did it then, he didn't know. He'd had the chance, and didn't take it.
He watched with the others, as the Master refused all of the Doctor's pleas to regenerate, finally dying in his arms.
It was as he saw the strongest man he knew completely break down, his anguished sobs filling the otherwise silent room, that Jack wished he had stopped it.
But he knew he couldn't have.
He couldn't save the man who had murdered innocents, who had killed Jack himself on a daily basis for the fun of it, and who the Doctor loved more than anyone else on the Valiant.
He just wished that he could have saved the Doctor. But he couldn't, and could only watch as the grief tore at the Time Lord like the bullet itself.
He just wished he hadn't let those chances pass him by.