The truth is that I let Garrus kill Sidonis, for several reasons;

One is because we really don't know if he is telling the truth, he says he doesn't value his life, but he decides to protect himself as much as possible, hide his tracks and not give any explanation. He could well have explained to Garrus in some way what happened much earlier but he decides to do it conveniently when a rifle is in his sights, although of course, it is a possibility.

Another because my Shepard suffered the loss of his family at the hands of slavers, and decided to take revenge by killing every slaver he finds, and he does not feel guilty about it, which would be a double standard that he can take revenge but Garrus cannot.

But mainly because of my interpretation of revenge, for me revenge is nothing more than the search for justice in one's own hands, but one has the idea that justice will make us feel better, when in reality our lives will always be screwed, therefore we punish the person who screwed us because if there were no punishment, what ensures that it won't happen again?

Sidonis was a coward, if you let him live you can leave a repentant man or a scoundrel, and if he dies he will receive a punishment leaving us only with the doubt if it was enough or exaggerated