Ex-VFR pilot here. I know enough to sit in judgement over this situation because it's crystal clear.
As is standard practice in this very congested airspace, the helicopter pilot asked for clearance to cross the regional jet's approach for landing using visual seperation. The ATC controller confirmed that the helicopter pilot had visual contact with the regional jet, then granted clearance.
The regional jet cannot maneuver for visual seperation while on approach for landing. It was entirely the helicopter pilot's responsibility to maintain seperation with the regional jet. The collision is entirely the helicopter pilot's fault.
Basically, the military just killed a bunch of civilians because they failed to do something that any pilot with an IFR license is regularly expected to do.