Gale was responsible for creating weapons that would trigger a second wave of bombs to target first responders and combat healers. This invention potentially ended up killing Prim, but even if he wasn't directly responsible, Katniss found this tactic so abhorrent that she lost her affection for him.
Before the Capitol battle, Gale had explained to Katniss his series of bombs designed to feed off human compassion: a small bomb would go off, and then when medical aid rushed in, a second bomb would go off, killing those people as well. Incidentally, it's this exact type of bomb that took Prim's life.
Though the rebels would go on to win against tyrannical President Snow and the Capitol, Gale's thirst for violent revenge seemed to sever any chance he had with Katniss: Mockingjay insinuated that Katniss' younger sister Primrose was inadvertently killed by a firebomb when working as a medic.
The double bomb that kills Katniss Everdeen's sister, Primrose, was designed by two rebels: Beetee and Gale, Katniss's lifelong friend. Previously, Gale had explained that the double bombs are designed to appeal to mankind's weakness for compassion.
Snow laughs after Katniss killed Coin because he enjoys the irony of the situation. He avoided the official execution ceremony, even though Panem's citizens still take their revenge on him, while Katniss ends up snatching away the victory and power that Coin thinks she has successfully secured.
The Life of Gale Hawthorne: Transformation Explained (Hunger Games Breakdown)
Why did Katniss kill Coin and not snow in Mockingjay?
In The Hunger Games series, Katniss killing President Coin instead of President Snow surprised many. Still, it was because Coin was revealed to be a ruthless leader with her own political interests.
Katniss searches his eyes for signs of emotion but only finds amusement in them. She remembers that they did indeed promise not to lie to each other anymore, and she realizes that Snow wasn't lying to her about Coin's role in the parachute bombing.
In the end, Katniss and Gale grew apart, as many childhood friends and young loves do. She needed something different in her life, and with Gale's link to Primrose's death, Katniss found a reason to cut ties. If Katniss ever wanted to find comfort again, she knew she had to return to Peeta for the long term.
Peeta, Annie, and Beetee vote no, while Johanna and Enobaria vote yes. After some thought, Katniss demands to kill Snow. Once she receives an agreement, she votes yes, for Prim. Realizing something is up (as Katniss would never do that), Haymitch votes yes, too.
Conclusion: Yes, Gale was indirectly involved with Prim's death, as in if he didn't create the bomb then she wouldn't have died. But what if someone else invented it? Gale did not kill Prim.
She later realizes, after a conversation with the imprisoned President Snow, that President Alma Coin sent Prim to her death (which was caused by a hovercraft by the rebels) both to sway the Capitol citizens to Coin's side by making them think Snow was bombing the children, and to swing Katniss to her side through the ...
According to the star behind Effie, Elizabeth Banks, she and Haymitch's Woody Harrelson improvised the scene. Considering she's been adamant about the characters having some romantic connection, it only makes sense that they'd share a goodbye kiss. It's very on-brand for Effie, as well.
It's only implied that they get it on via snuggling and other affectionate acts in close quarters — but if you have read the book, they really imply that they took their relationship to the next level.
Why was Katniss mad at Gale at the end of Mockingjay?
In the Mockingjay movie, Katniss confronts Gale about his role in Prim's death, which he struggles to come to terms with. Katniss, unable to look at him the same way, lets him walk away without a word and eventually ended up living with Peeta.
Even if Gale didn't intend to murder Prim specifically, he certainly intended to murder somebody. The two-phase bombs that he and Beetee invented were very clearly made with the intent of targeting medics and other civilians trying to save those wounded during the first phase of bombing.
Gale usually sold his hunting finds to the Head Peacekeeper, Cray. But today Cray wasn't there, so Gale got caught with contraband by the new Head Peacekeeper, who punished him.
Thankfully, Katniss noticed the clues. President Alma Coin presented herself as Panem's salvation in The Hunger Games, but several moments in Mockingjay hinted that she was an even worse villain than President Snow.
He knew something was seriously bothering Katniss because she had come to ask for his advice. But his 'boy troubles' comment put her off. So when he said 'I'm with the Mockingjay' it meant that he trusted that the Mockingjay (as the face of the rebellion) would do the right thing for the cause they had both fought for.
He only uses Katniss because the Capitol is brutally oppressive and he realizes that, with Katniss's help, the rebels have a real chance of bringing it down. This sort of cynical pragmatism largely defines Haymitch, and it stems from a combination of realism and a rebellious spirit.
The Hunger Games series, much like Harry Potter is for young adults, and the themes and motivations are not hard to understand. Katniss NEVER hated Peeta. She figured she'd have to kill him to survive and had no desire to like him because of that.
Gale admits to have fallen in love with Katniss in Catching Fire. He holds a hatred for the Capitol and has scars on his back from when he was whipped over 40 times in Catching Fire after being caught by the new Head Peacekeeper, Romulus Thread, illegally hunting turkey.
It is likely that he is a relative of Hilarius Heavensbee. In the Mockingjay - Part 2, Plutarch appears to smile as he looks at Coin's dead body, suggesting that he approved of her death.
After reading the book and watching the film, it seems clear that Lucy Gray realizes Snow was responsible for the death of his friend Sejanus Plinth (Josh Andrés Rivera). If Snow could sell his best friend down the river for personal gain, it was only a matter of time until he did the same with her.
Katniss speculates that Coin deliberately sent Prim to die in the Capitol bombing in order to fully gain her trust, but it's never outright confirmed or denied.
