People can go off the mental "deep end" for a billion reasons. A company cannot control everything.
I could find it cold in my cubicle and go insane shooting everyone in my building because the gunpowder explosions heat up the area. Is the company responsible for that?
In this case the employee was under high pressure at the work place. It is not a pure mental problem of this employee.
There are plenty of ways Amazon can handle this case better.