As the story of the fall of Abimelech continues, the writer of the Book of Judges, believed to be Samuel, concluded his remarks on why Abimelech died,

Thus God repaid the wickedness of Abimelech, which he had done to his father by killing his seventy brothers (Judges 9.56).

As with the other references in this particular narration, we conclude that the number seven here is merely incidental.