Of course keep the amount Nefario owes you and send back the rest. For all you know the script was meant to send you the remaining 10% but instead just redid the last payment amount.
Is that sure? Because i didnt get my payment again. So that means nefario took some of my coins as a fee or what? If not i should have gotten the doublepayment. So where did i say yes to this fee?
Why would he 'accidentally' erase the database? People might be able to come after him for lost bitcoins, but they can't come after him for the assets, especially unregistered, unregulated assets denominated in something not considered money. Nefario is the only one with the evidence. This whole claims process should have been done by someone else at BitcoinGlobal since Nefario pisses his pants when he even imagines a government regulator.
If such "accident" or anything happens to the database no one would believe it. Backups are common sense especially for websites handling money.
If this happens it would be clear why. And i fear the anger of people could be enough for him to fear for his life. I mean even now there is so much anger. How will it be when its clear he killed the database? I will not imagine.