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.
There are going to be alot of these this time of year. Near the holidays.
Alot of depression by people being lonely and everything comes crashing down from the year all at once at the end to deal with.