It's concerning that paybtc isn't explicitly stating that any payment will go first to repay principal (total deposits - total withdrawals) first before any "interest" is paid. In this case, the "interest" was just the btc of the new depositors.

In the event that paybtc did a straight percentage based payout across all his accounts just based on the current balance he would be running his own ponzi scheme.

This too could be the motivation in not handing over the details. The only reason to hide the details of a sub account would be if that sub account had withdrawn more than it had deposited and therefore revealed the location where other peoples btc went.