Also, while i feel it'll make for great drama, i should let you know that destroying the cave entrace is essentially a death sentence for the faunas inside. Remembered: they needed that dust to dig further in. The mine is much smaller than you may think. That and it would only take a few hours for them all to suffocate to death is you seal them underground. And yes, the guard is quite aware that this would happen, and is expecting it. If "gunning everyone down" didn't already tip you off on his attitude about the situation.

Basically, he want Team MODA to kill everyone in the mine.