In Mein Kampf (1924), as previously noted, Hitler stated that if the best men were dying at the front. the "least we could do was to wipe out the vermin.” If during the war

twelve or fifteen thousand of these Hebrew corrupters of the people had been held under poison gas, then the sacrifice of millions at the front would not have been in vain.

The following passage foreshadows the Final Solution:

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.

In June, 1941 (seventeen years later), Hitler began to fulfill this desire. The odds of this happening might have been about ten-thousand-to-one. 

Nevertheless, this did happen (in late 1941): Jews were held under poison gas and died, just as Hitler (and some other Germans) had been the subject of poison gas attacks and died in 1917-1918. Do unto others as had been done unto you.

The Holocaust, for Hitler, was the climax or fulfillment of the First World War. Now, Jews would not be exempt: they too would die as the result of poison gas.