Do you think I'm a bad friend?
Please don't think like that. Why do you feel like a bad friend?
I've been hanging out out with other people behind my friend's back
If you feel what you are doing is bad then you should try to change. Make some time to hang out with your friend too.
Nah, I'd rather hang out with other people.
Well, people do grow up and change, but it shouldn't be a reason to completely cut the friend off. Maybe ask him to come along with you with the other people so that he can also change to be the type of friend you want. This way, you are giving him a chance to be as cool as you, so you won't feel bad about your friend.