Why destroy bitcoins?
A non-malicious reason to deliberately destroy notable amounts of Bitcoins would be for a hypothetical alternative blockchain that recognizes Bitcoins sent to a special address as newly generated coins. One could for example try to establish a new "Altcoin" which is incompatible to Bitcoin, but every private key that signs a Bitcoin transaction to a special impossible Bitcoin address is eligible to also spend an according amount of Altcoins. This would enable a secure one-way conversion to a separate blockchain without the need for exchanges.