There's 2 kinds of evidence.

• Circumstantial evidence - relies on an inference to connect it to the conclusion (e.g. guy saying before hand he won't kill himself).
• Direct evidence - no additional inference/evidence is needed (e.g. video of a guy going up to the car and shooting him).

The guy saying he won't kill himself requires inferring that he's being truthful when he said it and that he didn't change his mind. It's not non-evidence, it does point to suicide being less likely. But it's far from conclusive. If there's no sign of entering the vehicle or that a struggle occurred, then I'd argue that far outweighs his prior statement.

They just happened to work at the same company and die right before they could testify on the same thing.

That's also a common misunderstanding, at least regarding the first (I'm not as familiar with the second). I'm a bit unclear on the details of the deposition - which side wanted it and was asking the questions, etc. (detailed here) but whatever the case, it was Boeing that demanded he come back for one more day. So if Boeing wanted him to not testify that day, they'd just send him home as originally planned. The only reason they'd do it then was to silence him generally...but doing it in a way that draws so much suspicion to them seems like an implausibly bad decision. Then again, it is Boeing. (Note that this is also circumstantial evidence, and requires assuming that Boeing isn't so dumb as to kill a witness in the middle of their own deposition, which may not be warranted).

Edit: corrected my own misunderstanding of deposition