It's possible to love more than one group. Thinking that the fan is betraying their fave for enjoying more than just them is frankly insane. If you really love your fave and want them to thrive, wouldn't you want to welcome more people into the fandom? Wouldn't you want to spread their music far and wide?
I really truly don't understand some kpop stans. If you find yourself so emotional about your fave that you get angry and lash out at people FOR ENJOYING OTHER MUSIC, it might be time to step back from kpop. Instead of it being an enjoyable pastime, you're clearly using it as a crutch to avoid deeper emotional issues.