Why wouldn't they have done that years ago before he had the chance to testify? He testified the previous week, so it's not like this prevented much...

Here's another source with more info:

Barnett filed a whistleblower complaint against Boeing in early 2017; his case against the company was heading toward a trial this June, his family said.

"He was looking forward to having his day in court and hoped that it would force Boeing to change its culture," the family said in a statement shared with NPR by his brother, Rodney Barnett.

The family says Barnett's health declined because of the stresses of taking a stand against his longtime employer.

"He was suffering from PTSD and anxiety attacks as a result of being subjected to the hostile work environment at Boeing," they said, "which we believe led to his death."

When John Barnett was interviewed by Ralph Nader in 2019, he said health issues had persisted after he retired from the plane-maker.

"It's taken a serious mental and emotional toll on me," Barnett said — but, he added, the safety of the airplanes rolling off the production line remained his main focus.

So he had been suffering from PTSD and anxiety attacks since 2017, and at least as of 2019 that was still happening.

Also, this happened after the first round of testimony:

Barnett's body was found in a vehicle in a Holiday Inn parking lot in Charleston on Saturday, police said. One day earlier, he testified in a deposition related to the string of problems he says he identified at Boeing's plant where he once helped inspect the 787 Dreamliner aircraft before delivery to customers.

If someone was going to kill him, surely they would've done so before he testified, no? Or maybe back in 2019 when he talked to the NYT?

Barnett, 62, made international headlines in April of 2019 when he and other former Boeing employees spoke to The New York Times about what he called shoddy manufacturing problems at Boeing.

It should absolutely be investigated for foul play since he was involved in court proceedings, but I think there's a simpler, more reasonable explanation that his history of PTSD and anxiety attacks pushed him over the end on the day we was supposed to go in for a second round of questioning. Questioning by lawyers can be very intense, especially if you're already suffering from a mental condition.