Was the Holocaust God’s punishment to the Israelites for rejecting his son?

No; in biblical narratives, the last punishment that God placed upon the Israelites as a “whole” would most likely have been the destruction of the Temple of Jerusalem by the Roman Empire in A.D. 70, (Matthew 24:1–2); and even this event can’t be traced directly to transgressions made by Israelites, but was more of a sign of the times (Matthew 24:15–25). You could more reliably assert that the invasion of Israel by the Babylonians, the Persians, the Greeks, and then the Romans all of which took place from 605 B.C. - 63 B.C. respectively, were a result of God’s wrath, mainly due to the Israelites engaging in political corruption and practicing idolatry. These events were prophesied in the Nevi’im of the Old Testament.

The Holocaust was more complicated than that. The Holocaust was one of many large-scale massacres that took place in recent history due to the lack of international treaties, peace agreements, and oversight.