If someone bought up 90% of the outstanding Bitcoins and destroyed their private key, the theory would be that the remaining BTC's would then be worth 10 times as much.
Supply is determined by the amount being actively traded. The market forces that dictate the price of bitcoin depend on the amount of supply and demand. If bitcoins are not being offered up for sale, then they play no part in determining the price. Whether the bitcoins were destroyed, or are just being saved for later, it makes no difference. But if someone bought up 90% of the bitcoins, the act of purchasing them would cause the price to go up incredibly high. After this large purchase, there would likely be a much higher demand than supply, which would maintain this high price; regardless of whether these 90% of bitcoins were destroyed or just saved away. The act of destroying them of course does not change supply, so it would not affect the price at all.