I’ve seen enough vindictive behavior in corporate structures that if assassination is on the table at all, so is assassination after the cat has been let out of the bag.
That said suicide due to extreme demoralization is also a plausible explanation.
You don't have to assassinate the whistleblower, but merely extra-judicially harass them as much as physically possible and let whatever happens happen.
Boeing may not have needed/wanted him dead, but managers have done plenty of petty bullshit retaliation before, for far far less.
That said suicide due to extreme demoralization is also a plausible explanation.