Under review
I'm going to swallow my words here: I had said that the master password will become a requirement for deleting a user.  This is untrue.  Anyone with access to your phone will be able to delete your user.

There are a few reasons for this.  Firstly, if you forget your master password or somebody adds their own user to your master password app, you should be able to easily delete those users from your app.  Secondly, any impediment I might add to delete the user from the device is trivial to circumvent by simply quitting the app and deleting it from the home screen instead along with all its data.

It is therefore important that you make sure you have a good PIN set on your phone and don't hand it out to others unsupervised: any unsupervised actor on your phone can do a lot more damage than just deleting your Master Password user!  And even if it happens, you should be able to just re-create it and your old passwords will be re-generated.