It occurs to me that this explosion may have occurred even if there were no lax inspections, faulty parts, etc.
The explosion was not the result of the faulty BOP. By the time anyone knew there was a problem which could cause an explosion on the deck, the BOP would be of no use as the methane would have already passed it.
After the explosion the BOP should have deployed to prevent the spill, but the BOP couldn't have prevented the methane bubble from traveling up the pipe. Someone would have had to know the methane was there, have their hand on the BOP control and deploy the BOP, closing the pipe at the sea floor before any methane traveled up. Likewise, the various safeties of the machines on deck were not designed to handle that volume or pressure of methane.
If the BOP deployed flawlessly, the rig would have still exploded and sank, only now we wouldn't have a spill.