What I don't understand (IF this suicide plot is true, which we don't know yet) is why the co-pilot didn't leave a spoken message, a sort of testament, an explanation for what he was going to do. He had plenty of time in the cockpit.
He was a pilot, he knew that soon after the crash SAR teams would've searched for the black boxes, he knew for sure how they work and the 30 minutes continous recording loop...I mean, you are desperate for some reason, you want to kill yourself, how could you do it without leaving a message?
That's why I'm more for a heart attack, or some kind of incapacitation...