It remained under control for the entire flight, landed hard, buckled, and broke open. There was a big fireball from spilled propellant, and a few large pieces of debris (likely COPV's - composite-overwrapped pressure vessels containing compressed gasses) were thrown a few hundred meters or so.
A failure of this magnitude was anticipated by the preparations they had made (clearing a large area around the pad, plus safety notifications to keep aircraft and ships out of the area); nobody was endangered.
The vehicle had a flight termination system (self-destruct) which would have fired if it had gone significantly off course before it left the safety zone.
The site is on the Texas coast just north of the Mexican border; if it had EXPLODED in midair, perhaps some debris could have ended up in Mexico, but there is literally no way this could have endangered anything in any other US state.
