For all the "Why would they do it instead of cashing out?!?!?" remember that a sufficently notorious theft could be tracked and the address-tree blacklisted. But destroying them makes your own stash more valuable, and makes any link between your profit and the destruction immensely difficult to prove.
Exactly. If you had a huge number of bitcoins then
introducing innocent-looking bad code into the official client to destroy a larger number of bitcoins would be a cheaper option than trying to take over the block chain.