A difficult question to answer, there are a series of different scenarios to be considered. To start with, what if Hitler had not existed, or had fulfilled, his initial ambition, and become an artist. It is most unlikely that there was a carbon copy of Hitler waiting in the wings, ready to promote National Socialism, so it is reasonable to assume, in this case, the Holocaust would not have occurred.
Supposing Hitler had been killed or died, prior to the invasion of Poland, as Hitler's deputy, Hermann Goering would have become leader, he was not a keen advocate of military action against Poland, and it is quite likely that the other prominent Nazi leaders would have supported Goering, had he decided not to attack Poland.
So WW2, would not have started on 3 September 1939, is it likely that it would have happened later, not necessarily, on the assumption that war did not take place, the persecution of the Jews would have been confined to Germany, and the removal of the Jews, dealt with by way of deportations [there were only 300.000 Jews left in Germany in 1939], so on this basis , the Holocaust would have been most unlikely.
What if Hitler had passed away after the occupation of Poland, well the Nazis would now have inherited a large number of Jews, round about 3.5 million. Goering would have become the leader, supported by Goebbels and Himmler, all these men were virulent Jew-haters, Goering the creator of the Gestapo, and architect of many anti-Jewish measures, Goebbels hated Jews, perhaps, even more than Hitler did, and Himmler was always a willing executioner.
The big question what next? well Germany would have been at war with Britain and France, so the Blitzkrieg in the West, was very probable, with a likely victorious ending. Following this, would the invasion of the Soviet Union have taken place, unlikely, since Goering was not in favour of Barbarossa, he tried to convince Hitler to give it up, but in vain.
There is not much doubt that mass murder of the Jews would have proceeded, with death camps in Poland, and Jews from Western Europe being transported East, so a reduced Holocaust would have resulted, but fortunately the death toll would have been considerably less, since the large number of Jews in the Soviet Union would have survived.