The Hunger Games, 2012 (Gary Ross) and The Hunger Games, 2008 (Suzanne Collins)
It's so interesting that, from the very beginning, Gale is established as someone who has a disregard for human life, and who does not truly view those different from him as "people." We see this here, where he dehumanises the children Katniss will have to fight in the Games. We see this in Gale's vocal resentment of Madge Undersee, in how he despises her for the privilege she was born with and grew up with.
This "us vs. them" infighting and mentality that Gale falls for (a mentality that the Capitol propagates, and that the Games act as a metaphor for) is one that eventually leads him to kill innocent children.
This thinking is also something Katniss must learn to break out of -- when she grapples with killing her opponents in the Games, who are truly just children, or when she realises "who the real enemy is" at the end of Catching Fire.
But Gale is unable to grow past this. He's willing to kill children (!) who are innocents, who haven't committed any crime other than being born in the wrong place at the wrong time -- just like Madge, who Gale despised in Book 1.
And Gale obviously DOES feel bad about "killing" Prim. But what's worse is the thought that, if it wasn't Prim who died, if it was just some random faceless people, would Gale still care?
I think Gale is a really interesting character whose development is so tragic yet still makes so much sense. It's a shame to see people write him off so quickly without seeing the analytical, thematic, and political value that we can gain by exploring his story and ideology.