I've been flying at night for some time, and from what I've seen the Washington D.C. accident isn't that hard to figure out. After flying over a presumably well-lit bridge, over a presumably dark river, the pilot could well have just flown into the water. I've been flying offshore, and after flying by a well-lit rig, setting up for a return, at 500 feet, looked up and we were at 200 feet in a descent at >800 ft/min, diving at the water. The other pilot was trying to fly visually, and just lost orientation. If I hadn't taken the controls, the helicopter would have impacted the water at cruise speed in a few more seconds.
Flying VFR-equipped helicopters, using VFR pilots, at night, will always result in accidents. It's simply stupid to do it, but companies are greedy, and the insurance companies can deal with the expense. Of course this gets passed on, but it's spread out, so companies just keep on keeping on. IMO the only way the current levels of deaths will decrease is if the insurance companies stop insuring night VFR flights. The FAA isn't going to do anything about it, that's certain.