While in medical school Marcel was already considered a thief. As a young doctor he was no better. It is believed that he killed three of his patients in Villanueve, France. Forced to leave the area, he moved to Paris where he continued his career as a swindler and a crook. During WW2, Doctor Petiot saw a golden opportunity to make lots of money. He bought a house in Rue Lesueur and customized it to become a sound-proof killing machine. There he killed up to 63 people, mostly Jews and others trying to escape the Nazis. The crafty Doctor told his victims that he was a member of the French Resistance and was able to arrange for their safe passage to South America for a steep fee. After receiving the money the doctor gave his victims a lethal injection saying it was a "vaccination" against foreign diseases. He would then lead them to a sound-proof room where he told them to wait for their Resistence escort. By then the poison would take over, the good doctor enjoyed watching their deaths through specially built peep-holes.
A thorough annihilator, he would mutilate the corpses and drop them into a lime pit. Later he started incinerating his ever increasing pile of dead. In early 1944 he was arrested and held briefly when he choked the neighborhood with the fetid smell of burning corpses coming from his incinerator. Policemen called to the scene found 27 mutilated bodies in the basement which, he said, were Nazis killed by the Resistance. Claiming to merely be doing his patriotic duty he convinced the cops to let him go. Free again, Marcel promptly disappeared.
After the war Parisians still remembered the stacks of bodies found in the Doctor's home. A newspaper accused Petiot of being a Nazi sympathizer and that the dead were patriots killed for the Gestapo. Wanting to clear his name the doctor sent a letter to the papers claiming that the Nazis had set him up and planted the bodies in his basement.
He was arrested again in October 1944 and charged with 24 murders. During his trial he claimed to have killed up to 63 enemies of France. No one believed his ties to the Resistance. He was found guilty on all counts and sentenced to death. The deadly doctor was guillotined on May 26,1946. It is widely believed that Marcel killed many more and dumped corpses into the Seine. Authorities believe that Petiot was also responsible for several dismembered bodies found in Bois de Boulogne near Paris in 1942.