NOW we are getting somewhere....
You are correct when you take a look at the extremes of the far right and the far left. As you say, how different are the two extremes when the 'far-right' of Nazi Germany are responsible for 7 million deaths and the 'far-left' of Lenin/Stalin are responsible for 20 million deaths? The bluring of the lines is quite considerable once you move to the extremes of either end of the political spectrum.
The truth is you really can't. But for purposes of studying the effects of extremism, we put labels on them so that we may compare and contrast the various political idiologies.
One way of determining whether or not a regime is far right or far left is their treatment of religion. With your far left regimes, atheism is the official party line, thus "godless communism"
On the far right, such as in Hitler's case, he wanted to exterminate the Jews, but he looked the other way in regards to christians. Contrary to manys' beliefs, Hitler was not an athist.
Hitler seeking power, wrote in Mein Kampf, "... I am convinced that I am acting as the agent of our Creator. By fighting off the Jews. I am doing the Lord's work." Years later, when in power, he quoted those same words in a Reichstag speech in 1938.
Three years later he informed General Gerhart Engel: "I am now as before a Catholic and will always remain so." He never left the church, and the church never left him. Great literature was banned by his church, but his miserable Mein Kampf never appeared on the index of Forbidden Books. He was not excommunicated or even condemned by his church. Popes, in fact, contracted with Hitler and his fascist friends Franco and Mussolini, giving them veto power over whom the pope could appoint as a bishop in Germany, Spain, and Italy. The three thugs agreed to surtax the Catholics of these countries and send the money to Rome in exchange for making sure the state could control the church.
Those who would make Hitler an atheist should turn their eyes to history books before they address their pews and microphones. Acclaimed Hitler biographer John Toland explains his heartlessness as follows: "Still a member in good standing of the Church of Rome despite the detestation of its hierarchy, he carried within him its teaching that the Jews was the killer of god. The extermination, therefore, could be done without a twinge of conscience since he was merely acting as the avenging hand of god. ..."
Hitler's Germany amalgamated state with church. Soldiers of the vermacht wore belt buckles inscribed with the following: "Gott mit uns" (God is with us). His troops were often sprinkled with holy water by the priests. It was a real Christian country whose citizens were indoctrinated by both state and church and blindly followed all authority figures, political and ecclesiastical.
Hitler, like some of the today's politicians and preachers, politicized "family values." He liked corporeal punishment in home and school. Jesus prayers became mandatory in all schools under his administration. While abortion was illegal in pre-Hitler Germany, he took it to new depths of enforcement, requiring all doctors to report to the government the circumstances of all miscarriages. He openly despised homosexuality and criminalized it.
In the end, does it really matter if Hitler was on the far left or the far right? Just like Mao, Lenin, Stalin, Mussolini and a host of other despots both past and present, he was evil personified and the world is a better place with him, and them, gone.