Sforno on Genesis 34:13/ספורנו על בראשית ל״ד:י״ג

1במרמה, they demanded that the people circumcise themselves expecting them to refuse. Alternatively, they thought that Shechem and Chamor, although prepared themselves to circumcise themselves, would not be able to convince the townsfolk to follow their example.

2וידברו, אשר טמא; they said that the kind of ipso facto voluntary circumcision which Shechem and his father were willing to perform on themselves was not relevant after Shechem had already defiled their sister. They considered this as in the category of אתנן זונה, offering the price paid to a whore as a sacrifice on G’d’s altar.