The flight crew’s decision to descend below the published MDA, cancel their IFR clearance to conduct an unauthorized circle-to-land approach to another runway while the airport was in nighttime IFR conditions, and the exceedance of the airplane’s critical angle of attack, and subsequently entering an aerodynamic stall at a low altitude. Contributing to the accident was the tower crew’s failure to monitor and augment the airport weather conditions as required, due in part to, the placement of the AWOS display in the tower cab and the lack of audible AWOS alerting.