In the ending of Mockingjay Part 2, Katniss shoots President Coin rather than Snow at the execution, and the crowd then turns on Snow. This is the key choice of the film, and in my opinion it is the clearest statement of what the series has been arguing since the first reaping.
Why does she do it? After the rebels win, Snow tells her that Coin staged the bombing in his mansion that killed the children and then the medics, including Prim. Coin, for her part, refuses the promised election and proposes one last Games using Capitol children. It could be argued that this makes the rebel leader a mirror of the regime she replaced. Boggs had warned Katniss, as he lay dying from one of the booby traps, to trust no one, including Coin, so the film plants the doubt well before it pays it off.
The shot also closes a loop that opened in The Hunger Games (2012), where Katniss volunteers to take Primrose's place at the reaping. Everything she did afterward, from the Victory Tour in Catching Fire to agreeing in Mockingjay Part 1 to become the Mockingjay on her own terms, was a way of keeping Prim alive. When Prim dies anyway, in a bombing that Snow says was staged by the side Katniss fought for, the war stops being about a cause and becomes a question of who gets to run the Games next. Perhaps that is why she turns on Coin and not on the man she has hated for four films.
The aftermath is quieter. Katniss is arrested, but Plutarch's letter, which Haymitch reads out, clears her and promises an end to the Hunger Games. The letter exists for a sad production reason: Philip Seymour Hoffman died on February 2, 2014, with most of his scenes already shot, and the two remaining scenes were rewritten rather than recreated with CGI. That choice keeps Plutarch's last words in the film without a digital double, which I think was the right call.
Katniss then goes home to District 12, where Peeta slowly recovers after being attacked by and then attacking her in the tar and sewer sequences. Years later they are married with two children, and Katniss plans to tell their story. Compared with the prequel The Ballad of Songbirds and Snakes, where a young Snow is shaping how the Games run, this ending shows the man's regime finished and the Games ended. The film closes the series, so it sets up nothing further.