Boeing doesn't order assasinations. If he was assasinated, it might have been someone at Boeing, who still has a clean slate but knows that the whistleblower knows something he did, and has the right connections from working for a defense contractor.
I wouldn't be so sure. It's well documented that the CIA take a "keen interest" in Boeing (and indeed that European security services do so with Airbus), undertaking corporate espionage and sabotage on their behalf. They consider aerospace to be "strategic" or some such.
