They were abandoning him. The wounded man knew it when he looked at the boy, who looked down, then away, unwilling to hold his gaze. (p.1)
The scene in which Fitzgerald and Bridger abandon Glass is the first thing we read in the novel, so that should clue you in that it's a pretty big deal. In our book, the most notable aspect of this passage is Bridger's reluctance. That will come to be quite important as the novel goes on.
Chapter 2: August 23, 1823
Glass's rifle was the one extravagance of his life, and when he rubbed grease into the spring mechanism of the hair trigger, he did so with the tender affection that other men might reserve for a wife or child (1.2.46)
Interestingly, Glass's beef with Fitzgerald and Bridger isn't about them leaving him to die—it's about them taking his equipment and, most importantly, his gun. On one hand, this is a practical consideration: survival would be a lot easier with the help of a rifle and a knife. On the other hand, this is as personal as it gets. After all, Fitzgerald hasn't merely taken Glass's rifle—he's taken the love of his life.
Chapter 5: August 30, 1823
When he walked in on Dominique plying her trade with the fat captain of a keelboat, the young man fell into a rage. He stabbed them both before fleeing into the streets. (1.5.11)
Like Glass, Fitzgerald is also a bit revenge-happy, which he further proves by stabbing a guy who beats him at poker at the end of the novel. There are some serious differences between the two men, however. While revenge is simply an impulsive, violent act for Fitzgerald, it's a moral imperative for Glass. He's a man on a mission.
Chapter 9: September 8, 1823
Fitzgerald and Bridger had acted deliberately, robbed him of the few possessions he might have used to save himself. And in stealing from him this opportunity, they had killed him. (1.9.40)
See, Glass can actually empathize with his enemies. He understands that they were in imminent danger, and he knows that they needed to protect themselves first and foremost. But by stealing from him, by taking the few tools he could use to stay alive, they made an active choice to hurt him, to "kill" him, as Glass states so passionately.
Fitzgerald and Bridger [...] were not mere passerbys on the road to Jericho, looking away and crossing to the other side. (1.9.39)
Here, Glass is referencing the biblical story of the Good Samaritan, comparing Fitzgerald and Bridger to the people who blindly walked past a robbed and wounded man and refused to give him any assistance. That's a classic 1820s burn.
Chapter 15: October 9, 1823
A gun. He appreciated Kiowa's willingness to equip him. What he wanted, though, was his gun. His gun and a reckoning from the men who stole it. (1.15.34)
Even after Glass reaches the relative safety of Fort Brazeau, he can't get his mind off revenge. Why not relax a little bit, dude? You just fought a bear, a pack of wolves, and walked and crawled for hundreds of miles. You deserve it a break. Yeah, that's just not Glass's style.
Chapter 21: December 31, 1823
He looked down at Bridger, and something unexpected began to happen. The perfection of the moment began to evaporate. (2.21.92)
Or not. As he lays the smackdown on Bridger, Glass comes to the sudden, unsettling realization that he's actually just assaulting a nineteen-year-old kid who's not offering any resistance. Not a good look. This, along with Bridger's clear regret regarding his actions, leads Glass to do something unthinkable: forgive.
The desire to shoot Bridger down nearly overwhelmed him. Having crawled toward this moment for a hundred days, the prospect of vengeance was now immediate. (2.21.80)
After a long and arduous journey, Glass finally locates his tormentors. Well, one of them, at least—a fifty percent success rate is nothing to scoff out. While Fitzgerald has fled from Captain Henry and the crew, taking a bevy of stolen goods with him, Bridger is about to learn firsthand why revenge is a dish best served cold.
Chapter 26: April 14, 1824
Glass could bear it no longer. He reached beneath his capote for the pistol concealed at his belt. He pulled out the gun and fired. (2.26.106)
As for Fitzgerald, things are a little more complicated. Old Fitzgizzle has joined the army since Glass has seen him last, and Glass is forced to settle his dispute in a military kangaroo court rather than in a fistfight. As you can see here, though, Glass chooses to creatively, ahem, circumvent the proceedings.
Chapter 27: April 28, 1824
"Why did you come to the frontier? [...] To revel in a moment's revenge?" [...]
Still Glass said nothing. Finally Kiowa said, "If you want to die in the guardhouse, that's for you to decide." (2.27.33-34)
After his failed assassination attempt on Fitzgerald, Glass is confused. Should he try again, likely getting himself locked up for good in the process? Or should he just let it go? Seriously? That's an easy choice, bro—we'd go with Option B all day, every day. Plus, if it makes him feel any better, Fitzgerald is such a jerk that someone will take care of him sooner or later.