I agree that Icarrus's post at #248 is one of the best summaries. My thoughts are that it is unlikely to be engine failure as there was no distress call. It would seem most likely to be a loss of control for one of the following reasons:
1) Ice contamination on control surfaces.
2) Ice affecting pitot static system resulting in inaccurate airspeed and altitude indications, essential in IMC and could lead to disorientation.
3) A loss of primary AI gyro, vacuum pump failure etc could result in immediate upset.
4) Pilot incapacitation. The passenger would not have a hope.
The Malibu is quite a hot ship. I have no doubt reliance would have been put on the autopilot for night IMC flying. The first three possibilities above could all result in the autopilot either disconnecting or deviating from its intended path. This would need prompt manual intervention with instrument rating skills to avoid rapid loss of control. The JFK junior PA-32 accident at Martha's Vineyard is a good example of disorientation and loss of control. I doubt we will ever know the real cause of this tragic accident as the plane will probably never be found.