Remember, Kylo’s trying to emulate Vader, so a red saber could also be interpreted as part of that phenomenon.
edit: Also, the Sith are really a cult of sorts, obviously some kind of ritual to it. Is it that absurd to suggest there would be some kind of tradition like red light sabers? It’s not just bad guys, but specifically Sith, and Kylo’s trying to emulate a particular Sith.