From the article . . .
In a series of findings, NTSB investigators concluded that the chartered airplane was flying dangerously slow on its final approach to the airport. The twin-turboprop King Air 100 crashed about 2 miles southeast of the runway on a cloudy morning.
In April, NTSB aerospace engineers released a detailed report that stated the plane had slowed to 76 knots, or 87 miles per hour, seconds before the crash. The plane's flight manual calls for a minimum airspeed in icing conditions of 140 knots.
This tells me the plane had more ice than the pilots were aware of. Likely they didn't react quickly enough to the decline in airspeed which dropped them below minimum speed and altitude.
Planes with ice are hard to control, especially at low speed. This is not pilot error that justifies filing suit, IMO. After all, the pilots died as a result of what happened. They weren't going to knowingly put themselves at risk.