As German Soldiers perished in the First World War as a result of poison gas attacks, so Jews should be held under poison gas
Richard A. Koenigsberg

The statement below "hints" at what was to follow. It contains Hitler's idea or thought that eventually generated the Final Solution.

Hitler (Mein Kampf, 1924): “If at the beginning of the War and during the War twelve or fifteen thousand of these Hebrew corrupters of the people had been held under poison gas, as happened to hundreds of thousands of our very best German workers in the field, the sacrifice of millions at the front would not have been in vain.”

Hitler was saying, essentially, that as German soldiers had perished as a result of poison gas attacks in the First World War, so Jews ("Hebrew corrupters of the people") should have been held under poison gas--and perished--during the First World War. 

It's not clear why the sacrifice of millions of men at the front would "not have been in vain" if Jews had died too. But apparently--if the death of fifteen thousand of Jews was worth the death of millions of German "workers"--the death of Jews must have been profoundly important to Hitler.