I guarantee you that a halfway decent lawyer would still find a way to sue you and win. Any lawyer could find a way to sue you and the ensuing legal scuffle would consume more money than you made on the project.
Just one of many points they'd raise - once the client pays, that kill switch is still present and could be tripped by the developer, the developer's employer, a hacker, or accidentally.
1> When you make time limited software: You provide a different version that's just the demo - killswitch for the final delivery after final payment.
2> You don't delete their data ever. That's their data. It's their work. Nothing can make it your work. Use of the demo to make their work can be an implicit acceptance of any contract making them pay you, but don't ever screw with their data/work.
Not if it was in the original contract.