Dead man's switch that uploads a message to YouTube that's basically "If you're seeing this then i have been assassinated by the company. I did not kill myself, I was determined to testify. Because I will now no longer be able to, what follows is my testimony" and then do a tell all. Could work it out with a lawyer in advance, too.
You could just have a server up and running, paid several months in advance, that did it automatically but you might want a human (i would think a lawyer) to also be involved.
Mate, it is extremely common for suicidal people to deny those thoughts. Here some stats, about 50 % deny it shortly before they actually do it. So saying "I will no kill myself" is really useless.
I'm not sure that's generalizable to this scenario. That's also pretty time constrained. Besides that, was there any evidence this person was depressed or otherwise suicidal? Maybe there is but i sure don't know if it.