After all Katniss had been through, two Hunger Games (one which was specifically made to kill her), the revolution, losing many people she cared about including her sister, who she loved above all others. she needed to believe in the good of human nature and have hope again
That’s why she married Peeta. He brought out a soft side in her, he made her feel safe and see that the world could be a better place. Gale, while he was a good friend, couldn’t give her that.He had grown too vengeful, and Katniss didn’t need that. Gale was hardened by the harsh world they lived in, he’s not the white knight I’ve seem some posts on here make him out to be.
That doesn’t make the house evil. They’re not the nicest but we do see examples of Slytherins who aren’t all that bad. No one in Potterverse is perfect. Slughorn is the one who rallied the troops and who came back to fight Voldemort. Harry would not have recognized the Slytherins who fought, he was pretty against making friends with them (a flaw on his part). Slytherins aren’t going to be knights in shining armor, they’re the ones who say “We’re outnumbered, I’m coming back for reinforcement” and who carefully plan and scheme and go for what they want. There’s something to be admired in that, too.