I changed my friend's netflix password.
That's pretty rude. Why did you change his password?
I found out he's been binging our favorite show on his own, and I think that's selfish.
I can understand how disappointed you are that your friend didn't wait to watch the show with you, but it is wrong to change someone's account information.  People are free to watch whatever they want, when they want.  If you wanted to watch a show with your friend, you should have been up front about it.
Whatever, I'm gonna make sure he doesn't touch that account ever again.
Sometimes friends like to have some time alone. If watching the show together is extremely important to you, you should let him know that, and not just mess with his account behind his back.