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From my point of view Occam's razor here wouldn't be assassination. It's would be as mentioned above, mental health and overwhelmed by the sheer impact of your whistleblowing.
Maybe there is no Occam's razor here if there's no obvious answer.
I hope this is just bad phrasing and will assume you mean just for this whistleblowers case. We don't know though.
Assassination AFTER the whistleblower information has been released and is now out in the open really doesn't make any sense to me though. As conspiracies go, its not even humorous.
That said suicide due to extreme demoralization is also a plausible explanation.
Boeing may not have needed/wanted him dead, but managers have done plenty of petty bullshit retaliation before, for far far less.