Frustration
Garrus couldn't hide the fact he was upset with Shepard. She got in front of his shot. Stopped him from killing Sidonis. He wasn't pleased at all. That moment meant everything to him. He had waited for the day when he could finally end that bastard's life.
And she got in his way and stopped him. Garrus knew it was because Shepard was worried about him. She kept mentioning that throughout the mission. Or maybe she stopped him because he knew better. She taught him a lot. She pointed out that there was nothing left of Sidonis. He was already dead, living with the guilt of being responsible for the death of Garrus' squad.
There would be no point in killing him. So Garrus decided to let him go. Sometimes he hated it when Shepard was right. But already he'd forgotten about Sidonis and at last let his men's memory go.
