Himmler played a central role in the Nazi policy of exterminating Jewish and other non-Aryan people during the second World War. He gave direct orders concerning the use of gas chambers in concentration camps. He actually deserves to endure the torture and horrors that he caused others to suffer, but at the very least, he should be tried and convicted of war crimes and executed.
Heinrich Himmler was a notorious war criminal and he should have been sentenced to death. He was responsible for the deaths of countless people, and he will be remembered as a brutal criminal. Although the death penalty is extreme, sometimes it is the only appropriate punishment for the worst crimes.
War crimes are monstrous affronts to all of humankind. The type of person who commits these crimes shows no concern for other people. Consequently, no concern should be shown for Heinrich Himmler. He should be thrown in a dark hole and forgotten about except for the bare essentials needed to ensure that he lives long enough to regret his horrifying actions.
As Adolf Hitler's interior minister, Heinrich Himmler was directly responsible for the Holocaust and rounding up Jews. Himmler most likely would have been sentenced to death for war crimes. Fortunately, he took it upon himself to ingest a cyanide pill shortly after he was captured by Allied forces in May 1945, obviating charges of war crimes.
I'm not a big fan of the death penalty for total monsters like Heinrich Himmler. Death is quick and we make it too painless. The Nazis certainly didn't make the deaths of millions quick and painless. Better to let people like that rot in a cell deprived of basic liberties for the rest of their lives.