Oh, I see what you're saying. He would have an incentive to sell enough coins to drive the value to almost zero by collapsing confidence in bitcoin permanently, such that he could walk away with the cash while still paying people back their worthless bitcoins.
That's an interesting idea, but it seems like a longshot to actually succeed. You'd have to drive bitcoins really close to $0 for it to work, and they would have to stay at that value even as he buys several more times of bitcoins back to cover the loans.
This scheme is much more difficult to pull off than simply taking the money and running. I think you underestimate the probability of him getting away with it.
Something
like this is certainly possible, if you're rich enough in BTC,
compared to the "real" depth of the orderbook down to your target price.
You don't need to drive the price to zero, or make it stay there for the scheme to work.
Extremely simplified hypothetical scenario with completely fictional numbers:Assume that for some strange reason find yourself in possession of 500K BTC, 0 USD.
Assume that it takes 500K BTC to - however briefly - drive the price to 1 USD.
Step 1: You sell 500K BTC from 10 USD down to 1 USD/BTC.
Let's assume you manage to get an average price 5 USD/BTC, this nets you 2.5M USD,
Step 2: Immediately place a bid for 2.5M BTC at 1 USD/BTC.
Price rebounds from 1USD/BTC. You now possess 2.5 Million BTC, 0 USD.
Step 3: (Optional) Pay off Bitcoin denominated debt, if any.
And... you're done! Buy an island and retire.