I was going to reply to bafanguy, but OvertHawk and Asturias have given you a perfect answer.
Yes, we could potentially deliberately crash a 'plane, but after Germanwings, that is highly - highly - highly unlikely. What is more likely, as OvertHawk suggests, is a family being kidnapped and held until the pilot - or cabin crew member - of the family smuggles something to leave on board that will cause a future severe problem to that aircraft.
Hence EVERYONE in contact with any aircraft must be subjected to the same rigorous security screening.