There maybe very rare occasions of a structural design fault making the aircraft unflyable but sadly in the vast vast amount of cases it comes down to pilot error.
Not always in a serviceable aircraft but in mishandling a situation where one or more failures have occurred.
Pilots may loose an engine then crash the aircraft because the engine out situation was not handled correctly.
That is why we have emergency checklists to make sure we follow the correct steps in dealing with an aircraft that has gone wrong.
something happened pretty quickly with very little altitude at a guess to cause a good pilot to crash? He didn't have enough air under the plane and time to sort it. There is always that delay period from something going wrong to the pilot working out whats wrong and reacting correctly to it.
I do think the radar traces and altitude readouts will be very revealing in showing the altitude the aircraft should have been in a normal climb and its descent point to impact as well as the speeds from the traces
Pace