I'm glad to see we have educated pilots/people on this board.   I agree with all above, one other contributing factor I just read is their was a broken layer of moisture at 6500' and freezing temp at that elevation right at the time of landing although ground temp was 44 deg.   (according to NTSB it was conducive to icing at that level)  Lets just say he descended through a pocket of moisture and didn't have his deice boots turned on, this would greatly increase his stall speed due to the small amount of ice on his leading edges.  Couple that with too slow approach speed, too much flap, possibly some distractions in the cabin, maybe Aft CG? and BINGO your in a stall/spin with no room to recover.  Again I agree it wasn't just one mistake accidents usually happen after a series of bad decisions.    The pilot could have diverted destinations for many reasons, he filed for Bozeman (much larger busier airport) and landed at Butte (less traffic/people) knowing he had too many people he probably thought he had less chance for a ramp check at Butte and who knows he might have been Irish (lots of Irish decent in Butte) and just wanted to land thier.   Also he was flying for the Owner and (with the economy) was told to put everybody in, not given a choice, just trying to make thier vacaction work out good for them! I'm rambling now but again GodBless the families!!