Why did Israel do such a thing? Adoni-Bezek knew why,
And Adoni-bezek said, “Seventy kings with their thumbs and big toes cut off used to gather scraps under my table; as I have done, so God has repaid me.” Then they brought him to Jerusalem, and there he died (Judges 1.7).
He knew that he simply reaped what he had sown.
Concerning the number seven, this appears to be an incidental reference to the number seven.