Persecution of the Jewish people goes back for centuries before Christ. Though many Christians have used the crucifixion of Jesus Christ as reason enough to treat this group of people very badly.
The early persecution began when there were more than just the Jews (the family of Judah), but of all the Hebrew nations. These were the 12 tribes of Israel (if you accept the Old Testament as history). Much of the persecution was because the Hebrew tribes grew and claimed land for the increase population. The people of the tribe of Judah grew the most, and therefore took more land.
By the time the Romans had conquered the area, there were mostly only the descendants of Judah that were accounted for. And, of course, the Romans held no good will to the peoples that they had conquered, including the Jews.
During the first thousand years A.D. "after Christ", the Jewish people spread throughout Europe. Most countries only allowed the people to be "money lenders", called "usury" in the Bible. Though most wealthy people made use of this trade, most people were taught to believe that it was a dirty trade, and was to be despised. Also because of the crucifixion of Christ Jews were held beneath contempt.
During the second thousand years many of the Jews prospered and became wealthy through their industriousness, and keen understanding of the financial systems of the various countries that they were in. By the 1800s and onwards, there were many Jews that had also become very well educated. By the 20th Century, there were many prominent and well-known Jews throughout Europe, Great Britain, and elsewhere, such as the US. But old prejudices die hard.
Hitler rose to power by playing the politicians just right, and then taking their positions from them. Germany was in a state of devastation after WWI. "Xenophobia" (fear of strangers) is a common occurrence among groups of people when they are "down and out". Every country in the world seemed to hold the Jews as lower class citizens, even the UK and the US. It was very useful for Hitler's popularity to find a "villain" to blame for most of the country's woes. And there were many people who actually believed that there was a caste system of races, and groups of people who were on a scale that proved their differences and incompetence.
Hitler did not find much resistance to his philosophy that Jews were a lesser people. And from there, with his devoted followers, he moved them into a "cause" of the world's problems. No one should allow themselves to think that other countries did not know about all this. There were pamphlets published in the US and other countries "proving" this belief. Because there was already so much antisemitism (meaning: Against Jews) feeling in the world, many people just didn't pay much attention. There were, of course, many people who did not believe this way, but had become afraid to stand up for their beliefs
There is a theory that when Hitler's mother was dying of cancer, her doctor, who was Jewish, was blamed by young Adolf, and that this was the reason for his intense persecution. I think that though Hitler may have found blame there, it was only an addition to something that worked politically for him. I am NOT saying that he did not, himself, believe it. I think it is clear that Hitler was a deep antisemite. But he manipulated the belief and found favor amongst colleagues and even people of science. This gave him whatever else he might have wanted to "purify" Germany, and later the world.
He did not just pick on the Jews, of course. Anyone that was considered "defective", also died at his order. People in programs for Mental Retardation or Mental Asylums were taken away, in secret, and killed. Their families were told that they were in a new facility where they were to be cared for and in the best of places. Hitler cleared out many of the prisons and jails, because, to be a criminal was a "defect", and they were killed.