1) Currently difficulty is at ~10,000 TH
2) Single 1 TH mining box cost ~$1000 to manufacture (reality is much less than 1k, but for easy calculation lets use $1k)
3) To destroy the bitcoin network you need to control 51% of the network.
4) To bring 10,000 TH (assume evildoer wants to be thorough) online it will cost:  10,000 * $1000 = $10million,  add on another $10millon for R&D,  $20million total.
5) Goldman saches makes ~$30million a day in NET revenue (same with most other banks)
6) It will cost a SINGLE bank HALF day's net revenue to wipe bitcoin out - less than their office supply budget.