I always kill him to earn Garrus loyalty otherwise he won’t make it in three.

You can actually spare him and still get Garrus Loyalty @Drew Barnette

I used to always save him but lately I kill him now as I think what would my Shepard do if she had been in Garrus' shoes? What if she had been the only one to survive the crash of the Normandy and you found out the crash was caused by a former friend/crew mate? She definitely would have wanted revenge and killed them.

Garrus doesn't need to dirty his hands killing a former-friend-turned-traitor. It won't bring even one of his team back or change anything. Sidonis says he tastes nothing and enjoys nothing since the betrayal. He's a guilty man just going through the motions of life. Spare him and let him continue to suffer his remorse.

I let Garrus have his revenge.