THE ACFT CRASH LANDED DURING A TOUCH AND GO LANDING. THE PLT SAID THAT THE ACFT WAS CAUGHT IN A WINDSHEAR OR VIOLENT DOWNDRAFT AND WOULD NOT CLIMB. HE DID NOT SAY WHY HE WAS DOING A TOUCH AND GO LANDING WITH 9 PASSENGERS ON BOARD. THE PLTS ACCOUNT OF THE ACCIDENT WAS: AFTER TOUCHDOWN I RAISED THE FLAPS AND SET TAKEOFF POWER. AT 103 KTS AND WITH 500 FT OF RWY REMAINING THE ACFT WAS LIFTED OFF. AT ABOUT 150 FT AGL AND CLIMBING I STARTED TO RAISE LANDING GEAR BUT THE ACFT STARTED TO SINK SO I LEFT GEAR DOWN. THE TREES WERE MOVING VIOLENTLY. I FELT I WAS CAUGHT IN A WINDSHEAR BECAUSE OF A SHARP DROP IN AIRSPEED AND THE ACFT QUIT PERFORMING. THE ACFT CONTINUED SINKING AND I DECIDED ON A CONTROLLED CRASH TO SAVE THE PASSENGERS. A WITNESS STATED HE (THE PLT) TOOK OFF WITHOUT ENOUGH POWER. I SAW THE PLANE WITH ITS NOSE HIGH ANDTAIL LOW AND I LOST THE PLANE WHEN IT WENT INTO THE TREES. THE ACFT HIT SEVERAL CITRUS TREES AT A SHALLOW ANGLE OF DESCENT 1200 FT FROM THE END OF THE RWY. THE ACFT WENT 150 FT THROUGH THE TREES AND 350 FT ON THE GROUND BEFORE STOPPING