THE NIGHT BEFORE THE ACCIDENT THE PLT & 2 PAX WENT GAMBLING. THEY RETIRED FOR THE EVENING (MORNING) BETWEEN 0315 & 0400. THEY AWOKE AT ABOUT 0830. AT 1007 THEY FLEW FM BULLHEAD, AZ TO BIG BEAR, CA. WHEN THE PLT ATTEMPTED TO DEPART FM THE BIG BEAR ARPT HE LEFT THE MIXTURE CONTROL IN THE FULL RICH POSITION. AS THE ACFT TRANSITIONED OUT OF GROUND EFFECT IT STALLED. THE PLT HELD THE YOKE AFT AS THE ACFT ENTERED AN UNCONTROLLED DESCENT & CRASHED 1880 FT BEYOND THE RUNWAY'S END. THE FRONT RIGHT SEATED PAX VIDEO TAPED THE ACCIDENT FLIGHT. THE 93-HR TOTAL TIME PLT HAD 33-HRS IN MAKE & MODEL ACFT. NO EVIDENCE WAS FOUND OF HIM HAVING RECEIVED DUAL INSTRUCTION IN HIGH DENSITY ALTITUDE TAKEOFFS. THE DA WAS IN EXCESS OF 8500 FT. THE ACFT'S CERTIFICATED MAX GROSS WEIGHT WAS EXCEEDED. A BOX CONTAINING MARIJUANA WAS FOUND IN THE ACFT WRECKAGE. TOX RESULTS CONFIRMED MARIJUANA IN THE PLT'S BLOOD & IN THE CAMERMAN/PAX'S URINE.