My candidate:
Josef Mengele
head doctor at Auschwitz, known as the Angel of Death
In one instance, he drew a line on the wall of the children's block 150 centimeters (about 5 feet) from the floor, and sent those whose heads could not reach the line to the gas chamber.
Mengele's experiments also included attempts to change eye color by injecting chemicals into children's eyes, various amputations of limbs and other brutal surgeries. Rena Gelissen's account of her time in Auschwitz details certain experiments performed on female prisoners around October 1943. Mengele would experiment on the chosen girls, performing sterilization and shock treatments. Most of the victims died, either due to the experiments or later infections. According to a website, "Once Mengele's assistant rounded up 14 pairs of Roma twins during the night. Mengele placed them on his polished marble dissection table and put them to sleep. He then injected chloroform into their hearts, killing them instantly. Mengele then began dissecting and meticulously noting each and every piece of the twins' bodies."[5]
Auschwitz prisoner Alex Dekel has said: "I have never accepted the fact that Mengele himself believed he was doing serious work — not from the slipshod way he went about it. He was only exercising his power. Mengele ran a butcher shop — major surgeries were performed without anesthesia. Once, I witnessed a stomach operation — Mengele was removing pieces from the stomach, but without any anesthetic. Another time, it was a heart that was removed, again, without anesthesia. It was horrifying. Mengele was a doctor who became mad because of the power he was given. Nobody ever questioned him — why did this one die? Why did that one perish? The patients did not count. He professed to do what he did in the name of science, but it was a madness on his part".[11]
http://en.wikipedia.org/wiki/Josef_Mengele