I’ve been seeing people vastly misunderstanding the whole point of the shootout scene, so I want to share my analysis here. It’s not just a redemption for Chishiya, and not just a way to make Niragi seem deep and complex. It’s a way to compare the characters of Arisu, Chishiya, and Niragi, and show their adaptability to change.
To begin, the initial main focus of it all is Chishiya. He has just left the King of Diamonds venue, and has realized that he is jealous of people like Kuzuryuu who are able to die happily with a purpose. He is seeking to speak with someone, to open up about his inner turmoils. That is when he gets caught between Arisu and Niragi, two ends of a morality spectrum he is struggling to find his place on.
Now, why does Niragi start this shootout? He says that he, Arisu, and Chishiya are all too similar, and due to this only one of them can live (although, it is later evident that it is for this very reason all three of them survive). Niragi doesn’t say they are similar in regards to their actions in the games; he says this in regards to how they are all inherently selfish people at the core. They put themselves first and foremost, whether intentional or not, and hurt those around them as a result.
For Arisu, this makes more sense in the context of his manga characterization. Manga Arisu has the unintentional tendency of doing things for his own sake when he thinks he has the group’s best interest at heart, and his speech mannerisms mainly revolve around his own feelings. He is selfish without realizing it. The shootout finally makes him conscious of this, and makes him want to remedy this. He is adaptable to change for the better.
For Chishiya, this has been evident from the beginning. He will manipulate and use others for his own gain, without worrying what happens to them afterward. He never goes out of his way to help others, but at the same time never directly hurts them either. He has and recognizes this selfish indifference towards all facets of life. However, the games make him envious of people like Arisu, who are adaptable to change.
For Niragi, it has always been obvious how he embraces his selfishness. He enjoys going out of his way to hurt others and make them suffer. He becomes the abuser to keep from being vulnerable. He is the oddball out of the three, because despite everything, he does not want to change.
Their selfishness is just the example that Niragi gives, but there are other things they have in common as well. Their upbringings, for one – they all lacked love and affection in their home lives. What makes them different in this sense is that Arisu had Chota and Karube to hold him up, who made him more friendly and optimistic towards life. Chishiya had no friends, but no enemies either, which led to his indifference towards life. Niragi had abusers, which led to his hatred and resentment towards life. There’s also how they all initially wanted to stay in the Borderlands; Arisu because he wanted to avoid his real world responsibilities, Chishiya because he was intrigued by the intellectual stimulation and death of the games, and Niragi because he could thrive in a world without laws. But they all eventually came to the realization that they wanted to leave; Arisu first due to escaping and finding a reason for living, Chishiya next due to realizing he needs to reject the offer to truly change, and finally Niragi not because he changed, but because he simply wanted to live.
As you can see, they are all similar characters at three different levels of intensity. Arisu at level one, the level where he is still capable of goodness. Niragi at level three, where he is too far gone and considered evil. Chishiya is stuck in a morally gray area between the two, trying to figure out where he should place himself.
Anyways, back to the shootout. Chishiya came here to talk to Arisu because he wanted to change for the better. But then Niragi came and pulled him backwards, halting his progress. The shootout is a battle of moralities; a push-and-pull between Arisu’s good and Niragi’s evil.
The whole point of this scene was to give all three of them time for introspection. Who are they currently, and who do they want to become? Niragi’s comment about all of them being similar spurs their answers. Arisu is disgusted at the realization of being so similar to Niragi, so he quickly puts his gun down and pledges to change. Chishiya is inclined to distance himself from Niragi once he sees Arisu’s resolve, and only puts his gun down to become uncharacteristic and save Usagi. And Niragi, upon seeing the other two take this chance to renew themselves, realizes that he was foolish to believe anyone could be like him in the first place. He truly is alone in this world, and he only has himself to blame for it. He never lets go of his gun – he fires. And in that split second, why does Arisu pick his gun back up after vowing to change? He does it to save Usagi. To not be selfish and shoot Niragi out of his own hatred, but for the sake of saving Usagi’s life instead. Saving Usagi is the turning point for Arisu’s character, and for Chishiya as well. Niragi’s attempt to kill her demonstrates how he never wanted to change to begin with.
So yes, Niragi was right in saying they are all similar in terms of their selfishness. But he was wrong to believe that they were kindred to him and could not change.
TLDR; all three of them are foils, bound together by their inherent selfishness. This shootout is the defining scene for their characters. Arisu is adaptable to change, Chishiya struggles with his alignment but ultimately chooses to change like Arisu, and Niragi was never meant to change in the first place.