This is actually a good idea.
And it already happens.
On my phone once it's unlocked and I want to use a certain app I still have to use a passcode for those apps.
If the photos of me killing someone are in the private photo app then that's under a passcode.
Sure I will open the iPhone with my finger. But you're not getting the passcode to the app.
What we need now is an app that automatically erases its data when the wrong code is used.
Of course I wouldn't have photos of a murder on my phone in the first place. That would be stupid but I can see there is a lot of stupid in this country based on photos put on the internet of people robbing banks/stores and then sharing it with the world on Facebook. LOL