The actual root?
Ishmael lived.
Sarai sinned then attempted to compound that sin with murder in a display of her lack of faith.
Sarah didn't believe that the God who gave her a child in her old age to produce a great nation would also protect that child.
Having first sinned in sending Hagar to Abraham's bed, she further had Hagar and Ishmael out in the desert to die.
The descendants of Ishmael are both a punishment to the Jews, and a light pointing to the one True God. For though the descendants of Ishmael are as grains of sand on the beach and the descendants of Isaac are few, still the Jews will prevail.
It is this existence of the impossible that ever drives those who would deny God to attempt to annihilate the Jews. Even if they are themselves Jews.
Ishmael lived.
Because someone didn't trust God and opted to diddle the maid, then sent her off once it became obvious he and his wife had screwed the pooch.