[(Genesis 38:1:) AND JUDAH WENT DOWN.] This is what Scripture says (Malachi 2:11): JUDAH HAS ACTED TREACHEROUSLY, AND AN ABOMINATION HAS BEEN COMMITTED IN ISRAEL. What is the meaning of JUDAH HAS ACTED TREACHEROUSLY? Judah dealt falsely, just as you say (ibid. 14): WITH WHOM YOU HAVE ACTED TREACHEROUSLY. Thus you say: Judah dealt falsely. Over what did he deal falsely? (ibid. 11:) FOR JUDAH HAS PROFANED THE HOLINESS OF THE LORD. Thus, JUDAH HAS ACTED TREACHEROUSLY, (ibid.:) AND HAS MARRIED THE DAUGHTER OF A FOREIGN GOD. And further on it says (Psalms 114:2): JUDAH BECAME HIS SANCTUARY, while here it says: FOR JUDAH HAS PROFANED. Thus, AND HAS MARRIED THE DAUGHTER OF A FOREIGN GOD.
Why Scripture Says Judah Acted Treacherously When He Went Down
Midrash Tanchuma Buber, Vayeshev 9:1
[וירד יהודה]. זש"ה בגדה יהודה ותועבה נעשתה בישראל (מלאכי ב יא), מהו בגדה יהודה, כיחש יהודה, כשם שאתה אומר אשר אתה בגדתה בה (שם יד), הוי אומר כיחש יהודה, על מה כיחש כי חלל יהודה קודש ה' (שם יא), הוי בגדה יהודה, ובעל בת אל נכר (שם) ולהלן הוא אומר היתה יהודה לקדשו (תהלים קיד ב), וכאן הוא אומר כי חלל יהודה, הוי ובעל בת אל נכר.