Ultimately, if you want to save Sidonus, plenty of people here have given you the method to do so.
Whether it's right or not... that's open to interpretation. But something to remember: if you let Garrus kill Sid, or let him go... you have a slightly different Garrus in the game now.
Either a more Renegade Garrus (got the job done) or a more Paragon Garrus (saw good in him.)
But after that point, it's hard to say if it's the same Garrus.
I know the Dr. Heart choice didn't have all that much effect on his attitude: Garrus just seems a natural Renegade. You can keep paragoning him 'till his ears bleed... and maybe now he gets the hint (if you save Sid.)
Personally? I never save him. (I did once for my Paragon, because, well, she's a paragon *read pansy

*) I think Garrus is there to get justice, not revenge, for his 10 other squad mates. And by paragoning the story from Sid's own lips, Garrus in the end does right by ALL his men. Sid would rather be dead from what he did.
So Garrus obliges him.
I don't see it as him self destructing or any other such thing. It's just a renegade thing to do. /shrug