"The meanest one is Rooster Cogburn. He is a pitiless man, double-tough, and fear don't enter into his thinking. He loves to pull cork. Now L.T. Quinn, he brings his prisoners alive. […] He will not plant evidence or abuse a prisoner. He is straight as a string." (2.37)
Mattie doesn't even have to think about this; she chooses Rooster, because she wants justice—her kind of justice, which just might involve not bringing the prisoner in alive.
They had ridden the "hoot owl trail" and tasted the fruits of evil and now justice had caught up with them to demand payment. (3.86)
Mattie feels sorry for these chained up prisoners at first, but she gets judgmental pretty quickly. It doesn't seem to enter her mind that some of these men are innocent, or that some of the crimes that the punishments they are receiving might not fit the crimes.
MR. GOUDY: I felt sure it would come to you with a little effort. Now let us see. Twenty-three dead men in four years. That comes to about six men a year. (3.160)
This attorney sees Rooster as a hired killer and as a menace to society. Mattie sees him as a bringer of justice. How do you think he sees himself?
He said, "You are a fine one to talk about looks. You look like somebody worked you over with the ugly stick." (6.150)
Some insults are just as good in 1875 as they are in the 21st century. Mattie gets what she deserves here, since she just made it clear to young Quincy that she thinks he's ugly too. Now that's justice.
"Ned was Quincy's friend, not mine. I would not blow on a friend. I was afraid there would be shooting and I would not have a chance bound up like I was. I am bold in a fight." (6.174)
Honor among thieves? Poor Moon is only about Mattie's age and totally in anguish over his physical pain and having to give up information about his associates. He's got a finely developed sense of justice, after all—so it's too bad he fell in with Ned Pepper's gang.
LaBoeuf said, "There is something in what she says, Cogburn. I think she has done fine myself. She has won her spurs, so to speak. That is just my personal opinion." (6.500)
Talk about justice: after teasing and needling her the whole time, LaBoeuf finally admits that Mattie has grit. That's the kind of judgment we like to see.
Rooster said, "Yes this is the famous horse killer from El Paso, Texas. His idea is to put everybody on foot. He says it will limit their mischief." (6.511)
Teehee. Rooster may just be teasing LaBoeuf about accidentally shooting Ned Pepper's horse instead of Ned Pepper, but he's got a point. If LaBoeuf hadn't prematurely fired, a whole lot of lives might have been spared, and maybe even Mattie's arm. Still, it's not quite fair to judge LaBoeuf based on this one incident.
I hurriedly cocked the hammer and pulled the trigger. The charge exploded and sent a lead ball of justice, too long delayed, into the criminal heart of Tom Chaney. (7.241)
Yikes. Check out that "lead ball of justice": this is pretty clear evidence that Mattie associates death with justice. She's not going to be happy until Chaney is dead—and she's not going to have to wait too much longer.