ספר בראשית

Or say , also, through the way [of interpreting] of remez (hint): behold, we see the deeds of people - evil people doing evil, the incitement of the evil impulse in them begins with saying to them that it is a mitzvah to do this, when in truth it is a transgression. And the tzadikim who safeguard themselves from transgressions sometimes will do a transgression for its own sake [lishmah], as we find 'it is permissible to lie [lit. change] for the sake of peace' (Bava Metzia 24a, Tosafot on Bava Metzia 24a:1:1, see also Yevamot 65b) - and this is "these are the generations of Yitzchak"(Gen. 25:19), meaning, the judgment that comes through transgressions. The reason of the transgression is because "son of Avraham" is a hint to a mitzvah, that is, that the yetzer hara says to him that this is a mitzvah, and there are tzadikim who do a  transgression for its own sake, as explained. And this is hinted in "Avraham begot Yitzchak", meaning, a great tzadik sometimes begets "Yitzchak", that is, Judgment, by sometimes doing a transgression for its own sake because of the tzadik's great righteousness. And this is easy to understand.