They turned the MCAS system back on against the protocol and never flew the plane by hand. They tried, but the auto-throttle had the throttles firewalled and when they re-engaged the MCAS the plane flew into the ground near the speed of sound, far above the “never exceed speed”. The plane was doomed by bad piloting. Boeing is not blameless here but bad pilots flew the plane into the ground by violating directives.
Oh, and systems knowledge is essential- turn it off and if necessary pull the circuit-breaker (and know what other systems might be affected.