It's important to remember that whistleblowing is extremely stressful, so much that it's one of the main things the government talks about on their whistleblowing site:

Practice self-care and stress-reducing activities throughout your whistleblowing process. It is common to experience toxic forms of retaliation – from professional isolation to gaslighting (manipulating someone by psychological means into questioning their own sanity) – which can lead to post-traumatic stress disorder, depression, or even thoughts of harm.

https://whistleblower.house.gov/sites/evo-subsites/whistleblower.house.gov/files/whistleblower_survival_tips.pdf

Researchers have found the same thing, being a whistleblower is terrible for your mental health:

About 85% suffered from severe to very severe anxiety, depression, interpersonal sensitivity and distrust, agoraphobia symptoms, and/or sleeping problems, and 48% reached clinical levels of these specific mental health problems. These specific mental health problems were much more prevalent than among the general population.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6604402/

In addition, "Half of Patients With Suicidal Thoughts Deny It"

Not only did approximately 50% of people with suicidal thoughts deny having those thoughts, roughly 50% of people who had died by suicide, and 30% of people who had attempted suicide had denied having suicidal ideation in the week or month beforehand.

Furthermore, in many cases, people who had disclosed in apps and on paper that they had thoughts of suicide then denied that they had suicidal ideation when questioned directly in face-to-face assessments or interviews. For example, in one study, nearly 60% of those who reported their suicidal ideation on an app then denied their suicidal ideation in a telephone interview less than 24 hours later.

https://psychnews.psychiatryonline.org/doi/full/10.1176/appi.pn.2021.10.9

So, just because he denied he was suicidal doesn't mean that's necessarily true. He might have been trying to appear strong to everyone while suffering in silence.

This should definitely be investigated as possibly being murder. And, even if the investigation does determine that he shot himself, they should keep looking to see if he was being blackmailed or if he might have been pressured into suicide.

I just can't imagine an executive at Boeing going out and hiring a hit man. But, what I can imagine them doing is hiring a team of private investigators to go through this guy's entire life and dig up every bit of dirt on him. It could be they found something really embarrassing and were going to blackmail him with it. It could be that they found something innocent that they could frame as being awful, like to make him look like he was a child molester or something.