לא תעשו לכם אלילים. כְּנֶגֶד זֶה הַנִּמְכָּר לְגוֹי, שֶׁלֹּא יֹאמַר הוֹאִיל וְרַבִּי מְגַלֶּה עֲרָיוֹת, אַף אֲנִי כְּמוֹתוֹ, הוֹאִיל וְרַבִּי עוֹבֵד עָ"זָ, אַף אֲנִי כְּמוֹתוֹ, הוֹאִיל וְרַבִּי מְחַלֵּל שַׁבָּת, אַף אֲנִי כְּמוֹתוֹ, לְכָךְ נֶאֶמְרוּ מִקְרָאוֹת הַלָּלוּ; [רש"י ויקרא כ"ו א']
אל תעשו לכם אלילם YOU SHALL MAKE FOR YOURSELF NO IDOLS - This command is repeated here with reference to one who has sold himself as a slave to a non-Jew in order that he should not say "Since my master engages in forbidden sexual relations I will be like him; since my master worships idols, I will be like him; since my master desecrates the Shabbos, I will be like him", on this account have these verses (v. 1 and v. 2) been stated.
How could one POSSIBLY think that just because his master does terrible aveiros - he is permitted to do them as well?
The Baalei Mussar explain: Man has a tendency to be WEAK and follows the crowd [Rambam Deyos 6-1] - especially people to whom he feels subordinate. This, he is liable to tell himself the story that he is permitted to do these aveiros b/c his master is doing them.
