Please explain what the reason was for slaughtering tens of millions
Well, you see, it all started with the assassination of Archduke Franz Ferdinand by a Serbian anarchist in Sarajevo ...
Pertaining to the slaughter of millions: After the end of World War II there were perhaps millions of German ethnic civilians killed in Europe partly out of revenge due to their forced removal back to Germany. Plus about four million were sent to Russia.
A little bit of forgotten history. In fact, it is entirely possible that more Germans civilians died after WWII than Jews killed by Hitler.
Your question is a non sequitur. There can be and have been many reasons for wars throughout human history. Wars, by their nature are ugly and people die. If you can clarify your question I will try to answer.