There is a long list of errors and omissions, which contributed to these accidents, but the bottom line seems to be software development malpractice. MCAS system was designed by Boeing and certified by FAA to keep operating with wildly diverging data from two AOA (angle of attack) sensors. Reports indicate that AOA was the only sensor type MCAS was using. There have to be at least 3 AOA sensors to have some level of reliability in case of a single sensor failure. MCAS should turn itself off in case of large difference between two AOA sensors readings. Whoever wrote and certified that code bears at least some level of responsibility.