I like to think a common misconception about any sorting is most people have the capacity for any of them should they feel comfortable in that setting. As far as my story went, that's where he was most comfortable at the time. But if you want a more in-depth reasoning, I thought it was important to showcase the perception of light and dark, good and evil, adding nuance where there isn't much in a world that sees everything as black and white more or less and views Slytherin as the worst of the worst. Could Keith have been a Gryffindor because he's brave and hot headed and cares about justice no matter what? Sure. He could have easily been a Hufflepuff, too, because he's fiercely loyal and values the relationships he creates, although not to the same extent as, say, Hunk and Lance. Pidge could have easily been Slytherin because she's cunning, and if I have any regrets, it's that I didn't put her there to support Keith better, but I digress. Keith is in Slytherin because he needed to be the proof we are what we choose to be above all else. Just like he's canonically half Galra and chooses to fight for the good of the universe. And I also personally believe it's where he fits best. Hopefully that answers your question, but if not, feel free to ask away.