As DM I'm a player as well. If I can't get behind a campaign theme, I don't see how I can run an effective or enjoyable game. While I don't ban CN characters like
@Helldritch , I do point out that chaotic insane is not an alignment and that I expect people to bring characters who are willing to work with the group.
Which is why I make all this clear when recruiting/inviting people to play my game. My idea of fun doesn't include PCs backstabbing each other or being the bad guys. I've never had an issue finding or retaining players, so I'm going to run a game I have fun running. If that means I'm not the right DM for every person that's fine. Some DMs haven't worked for me in the past either.