Pilot assisted suicide came to mind first. I agree with others, pilots outside of 1st world countries tend to be decent FMS managers, but really lousy pilots when the automation fails or is turned off. They have a history of turning a minor issue into a terminal issue just by clicking the autopilot off or by leaving it on too long when it’s malfunctioning because they know their chances of survival are slim once they turn it off.
Even if the elevator jammed they could still control it with stabilizer trim, if they knew that. The photo of it going straight down suggests the stabilizer was jammed in a extreme nose down position, the autopilot commanded a hard over and they wouldn’t turn it off, they had runaway pitch trim nose down and didn’t know how to turn it off, some other major mechanical malfunction, or pilot assisted suicide.
Only time will tell, maybe.