Why didn't Shechem understand why they insist that everyone circumcise? He should have understood that it suffices for him and his father's house to circumcise [and they insist that everyone circumcise in order to kill them]!
Reishis Chochmah (Sha'ar ha'Ahavah 4), Chochmah u'Musar Vol. 2, p. 462: His desire [for Dinah] made him err to understand their words simply.
Refer to 34:14:152:1 and 34:14:152:2.
Ohr ha'Chayim #2: In order to be Am Echad, all of them must circumcise.
Ha'amek Davar: They said that Shechem defiled her and desired her only due to lust; they do not agree to this. They consent to Chamor's idea, to become a unified nation.
What does "Ach" exclude?
Malbim: Chamor offered six gains if they will give Dinah (refer to 34:10:1:2). We consent to be one nation and live with you, and remain like strangers regarding the other matters. 1
Malbim (to 34:17): This was part of the enticement for them to circumcise. Bnei Yaakov will remain, intermarry with them and pay much taxes - "Mikneihem v'Kinyanam... ha'Lo Lanu Hem" (34:23)!
