SpaceX clearly has a winning method, as they have a success rate of 99.3% and hold the record for most consecutive successful launches at 253. Second place is Soyuz-U, which had 112 consecutive successful launches and an overall success rate of 97.3%. SpaceX has more consecutive successful booster landings (173) than any space program has had successful launches.
It exploded while attempting to test its landing maneuver. If it just turned off the engines and dropped itself into the ocean like a normal booster, it likely would not have exploded (for one thing, there wouldn't be any fuel left in it being saved for landing).
A few people have slosh modeled the booster flip around duplicating observed speeds and accelerations and surmise the following. The flip around was too rapid causing slosh to one size of the booster. In addition there was negative acceleration that would have allowed the remaining fuel to experience zero gees pulling it away from the bottom of the rocket. When the engines fired back up it would have hit the bottom of the rocket with something between 20 and 100 tons of force. Based on SpaceX description, the FTS was not used.
The second stage at least appears to be oxygen leakage. The graphs shown on flight day started showing a more rapid depletion of oxygen after second stage lighting. SpaceXs language was something like 'flight was unable to reach orbital insertion and was terminated, pointing to the FTS being used.
The second stage exploded as well, possibly because of FTS, possibly not (was it ever confirmed?).