In the ending scene there is a line that says Martians reward you for bringing peace. You can't write it off as Offscreen Villainy if Mr. Natas' is still respectable enough for the protagonist to work for him. He is a Villain with Good Publicity but then how the Martians know he is a bad guy?
And if Martians exist and can converse perfectly with humans why does he hire humans from so far away to mine?
This much is easy enough. If the Martians know Natas is evil and Natas knows they know it (and aren't agreeable to him or it), he'll obviously want to find people easier to dupe for whatever plans he has.
The garbled transmissions from killed miners; they could be Mr. Natas' work, which is again unexplained, or just natural hazards.