With respect to the most people murdered in the shortest period of time, I don't think you'll find anything to match the death camp at Treblinka, which saw 800,000-1,200,000 people killed in 15 months of operation. The total combined casualty numbers (killed, wounded, missing) for both sides at Stalingrad approximately equal this number. There is a fantastic biography of Franz Stangl, the man in charge of the camp: