Duty
He didn't look to his right where she was. He didn't want to see the hurt and anguish in her eyes. He might as well have though. He knew it was there. It ate into him, causing his guts to clench and his throat to tighten, but he couldn't let it stop him.
"Get to it, Major," he ordered sharply and walked away before her protest could reach him. He didn't have to have her approval. And he didn't have to glance back to know she'd already moved to obey his order. She'd do the job even though she thought it was wrong, worse than wrong even. She always did, and he knew what always being the good soldier cost her. One day soon, she'd either turn in her resignation or turn into someone who could do the job without hating herself for it. Whichever way it would unfold, he dreaded that day even as his words and actions brought it closer and closer.
The tyranny of command. He was the one who had to look at the whole picture, had to make the call for the good of all mankind, had to give the order. She was the one who had to carry it out. It was her unbelievably intelligent mind that would have to reconfigure the bomb, her hands would be the ones that programmed the coordinates into the computer, and it would be her quiet voice tight with unshed tears that would say, "Ready, Sir," just a fraction of a minute before her reluctant finger would press the button to his response of "Do it." He'd make the choice, and she'd pay the price in currency he didn't want to contemplate.
And if he hated himself for what he did to her in the name of duty, it didn't stop him from giving the orders. The war might not be won, but one more battle would be...and if they couldn't stand what they'd done to win it, too bad. He couldn't help that. He was a good little soldier, too, and he'd follow orders and fight the war in any way he could because losing it was unthinkable. Better they both lose their souls fighting for the good of mankind rather than swallowed up by the evil of the Goa'uld.
So when the time came he'd say, "Do it, Carter," and he'd screw up every ounce of determination he could find and meet her eyes with all her pain, horror, and reproach blazing through them so she would know he'd counted the cost and deemed it worth the price. He'd stand at her shoulder and give her what comfort he could with his presence while her finger hit the enter code and her eyes unwillingly tracked the devastation she'd wrought at his command. And later, when they'd returned back through the Gate, he'd let her rail at him with angry accusations that he should have found another way. And later still when her anger was spent he'd say, "Come here" and hold her as she cried. And when even that was past, he'd coerce her to eat and order her to sleep though he'd know better than anyone that neither would help.
And when she showed up at the next mission briefing and took a seat instead of throwing her resignation in his face, he'd be torn between weeping and breathing a sigh of relief.