You want to know who have that same mentality?
Terrorists.
Self-righteous assholes who think they're doing the world a big favor for cleansing the world and its people by justifying violence for their cause. They also think they're good people fighting for the right reasons. That's how delusions of grandeur work. They also think the world is against them for some reasons, and that existing rules and regulations do not apply to them anymore because they have their own brand new set of law and order. They also think they fight for justice and for the good of their people. And if innocent people were killed, its accepted casualty.
Garrus thought he was doing the right thing being Omega cleaner and scaring the bad guys away but it never occur to him that he was the bad guy. Everyone who wanted Garrus dead are also justified. Garrus didn't just mess up their operations. The leader of Eclipse want Garrus dead because Garrus killed his brother. The leader of Blood Pack and Blue Suns want Garrus dead because he tried to kill them for no reason. And their deaths accomplish nothing to these organization except creating a power vacuum on Omega which allowing Talons taking over and Cerberus to infiltrate easily and the gang itself found someone new to replace them. And Sidonis was his friend and he was physically and mentally tortured to reveal their base... and Garrus simply want to put a bullet in his head without even talking to him or investigate whether it was the truth. He just want payback to make himself better because he couldn't stand that it was directly his fault that everyone died.
And did regular Omega people worship Garrus in ME2 even in Omega DLC? Let me answer this for you : Zero.
So tell me again what brand of justice that Garrus adhere to when he wasn't sure about it himself that he needed Shepard as is moral compass.
About Sidonis, I do agree that he should've investigate first, but it's easy to say that when the thing didn't happen to you. Even without going with real life examples, in DAO I never let Loghain live when I roleplay elven characters, because of his deal with Tevinter on selling elves as slaves. Or putting Bhelen to the throne as a noble dwarf. The events my characters suffered shape their decisions later on. The same goes for Garrus. While my Shepard wanted to stop him and listen to Sidonis' version, I don't condemn him for what he wanted to do.
Also, it's not his fault what happened to his squad. Regardless of the opinion on his stint on Omega, he didn't force anybody to join his squad. They all volunteer.
I really like Garrus, but I do recognize his flaws. You seem howewer to go in the opposite direction of Hound. And it should be nothed that Omega isn't exactly a normal place to live. The whole station is controlled by criminals. Innocent people don't really have someone who protect them from criminals. Omega has no law or regulations, so the comparison you made isn't exactly accurate.
Garrus was certainly naive in thinking that he could change Omega, but it wasn't a bad thing either. The location should be taken into account when judging his actions. It's not like he started being a vigilant on the Citadel, where there are organization that fight (not in a efficient way) crime.
I'm kind of curious to see your opinion on Batman/superheroes in general.