“The purpose is to eliminate evidence.”
So the tunnels would be like large mass graves. The Nazis used mass graves for a while, and then switched to cremations. I guess the tunnel approach would serve a double purpose, a killing venue and disposal site in one. But who would they be hiding the evidence from? If foreign observers/invaders reached the site, they presumably would find the tunnels. If they didn’t, there is no one to hide things from. Maybe the intent is to hide the evidence from other prisoners. Or maybe they don’t have any other use for slave labor, so digging tunnels is a make work project.
It is still better to get rid of live witnesses. Tens of thousands of survivors giving their testimony would leave the regime absolutely no wiggle room. If there are only dead bodies, they could theoretically pretend that they are victims of plague or something.
The leadership needs time to disperse. By forcing South Korea, or whoever, to have to dig up the tunnels to find the bofies, the leadership is hoping to be long gone and be inaccessible in China.
I think it’s a sign that North Korea’s end is drawing nigh, if the leadership is planning to kill everybody.
Nature reclaims its own....