I don't know why Boeing would wait 6 or 7 years to kill him, right in the middle of his deposition, at the exact moment it would greatly intensify exposure of the story.
I hope people focus on the facts he was sharing-- that Boeing was taking safety shortcuts and ignoring clear risks to keep production deadlines--and what that means about possible risks to the millions of Americans that fly, not fall off into conspiracy land.
Pretty sure no one at the top of boeing cares about anything other than the next quarter, and that mentality is exactly the one that would panic and pull the trigger.
They didn't kill him for 6 or 7 years because boeings weren't falling apart in the air back then. Now that its in the news they knew his testimony would sink them, so they made a gamble.