Ok after rereading the preliminary report, ATC gave them a vector north and clearance to a named fix. They were not off course. While on a vector clearance, terrain and obstacle clearance is ATC's responsibility.

Once the accident flight requested and received a visual approach clearance ~30 miles out, terrain and obstacle avoidance becomes pilot responsibility. It seems likely to me that whether GPS was working or not, whether RNAV 24 or ILS 24, they'd have requested a visual approach. And the pilots did not adequately account for the terrain between the flight and the airport when they initiated the descent, and descended below minimum safe IFR altitude.

Further due to this flight operating under 14 CFR 135, there's a regulatory bridge to bind the flight to the (Part 135) Certificate Holder's operation specifications (OpSpecs). Pilots cannot operate outside of OpSpecs.

It's possible the medivac company's OpSpecs require mandatory direct visual contact with terrain and obstacles - something that could not be achieved with 0% illumination due to no moon. Some OpSpecs will prohibit visual approaches at night to non-towered airports. SRR airport has no tower. I'm not familiar with this operator's OpSpecs.