The Hunger Games: Mockingjay - Part 2 is a solid, final franchise installment, and one of the more interesting and meaningful scenes is when Katniss kills President Alma Coin, causing Snow to eerily laugh. The film picks up after Mocking Jay - Part 1's unsettling, cliffhanger ending where Peeta (Josh Hutcherson) was thrashing in aggressive, brainwashed distress, and the stage was set for the series' conclusion. The rebellion against the Capitol, led by District 13's President Coin (Julianne Moore), finally gains traction and eventual success. Former President Snow (Donald Sutherland) is imprisoned and set to be executed by none other than the Mockingjay herself, Katniss Everdeen (Jennifer Lawrence). Of course, when it's time to let the arrow fly, Katniss' aim is true (as Coin tells her: "Mockingjay, may your aim be true as your heart is pure"), just not in the way that the leader was expecting.
Katniss has to kill Coin instead because she sees through her projected facade of wanting a better, free Panem. She sees the truth: that Coin is just another dangerous person of power with her own agenda, looking to selfishly capitalize on what a populace wants by pretending she shares their exact ambitions and convictions. Katniss realizes that, though Snow should answer for his atrocities, Coin is set to take his place and be the new leader who perpetuates the darker and more self-interested elements of human nature. Not to mention, the new, interim president was behind Primrose Everdeen's death. In another act of rebellion, which she became so famous for over the course of the franchise, Katniss decides to kill the character who's become the newest figurehead of evil — showing that Panem will no longer accept morally bereft and double-dealing leaders.
All of this makes sense, and even does a great job of layering an abundance of meaning into one fairly quick scene. The mob of long-oppressed and exploited people kills Snow — a fitting end for his character. But, before his death, after Katniss kills Coin, Snow breaks out into a fit of laughter. He knows that he's still going to meet his demise as well, but the twist gives him great satisfaction. The moment stands out as odd. After all, he's still lost and he's going to die one way or another, even if it's not by that specific arrow.
Though audiences are never as privy to former-President Snow's inner workings as with other characters, there's a handful of possibilities for this. For one thing, Snow is a main villain. He's been the ruler of the country that violently pins its teenagers against each other for sport, and he's a glaring symbol of oppression in this class divide-driven series. The fact that he escapes death, even if momentarily, still gives him a sense of getting away with some of what he's done. Instead, his enemy, who he himself has helped turn Katniss against, takes the heavily symbolic arrow.
Snow laughs because he enjoys the irony of the situation. He avoids the official execution ceremony, even though Panem's citizens still take their revenge on him. Katniss ends up snatching away the victory and power that Coin thinks she has successfully secured; Snow is a rather cerebral character and must have a healthy appreciation for potent irony. Katniss was the face of Coin's rebellion in Mockingjay - Part 2--the movie's mythologized, titular hero who starred in the propaganda films to fuel the effort's fire. Coin had no choice but to work with Katniss, but in the end, the poster girl turns on her anyways in an unforeseen act of defiance. Snow's character always embodies a sort of sinister, calm crypticness, and it's fitting that the last thing he does before his death is darkly and smugly crack up at an act of violence.