Maybe he didn't want it to look like a suicide. Maybe he was trying to create a credible decompression/hypoxia accident scenario, starting the descent but then passing out, and hoping that any evidence to the contrary would be destroyed in the high-speed collision with the mountainside.
If any of that was really true then surely he would have just acted like the pilot of Silkair 185 or MH370 by pulling the CVR and FDR circuit breakers to stop both data recorders before he initiated any form of suicidal action or anyone was beating on the cockpit door with all of that being logged for crash investigators.
Also surely if he wanted to die but was still hoping to trigger a big life insurance payout for his dependents his best bet would have been to turn off the CVR and FDR and then de-pressurise the airplane and let it fly on its original autopilot track till it ran out of fuel and ran in to a hill or mountain on its own, just as the ill fated Helios jet did. Although as this wasn't a 737 he couldn't have used the pressurisation switch in Manual mode dodge and would have had to de-pressurise the cabin deliberately.
Still hard to figure though why someone wants to kill another 149 people deliberately and yet not tell the world at large their motives for doing so.