Why is Edom compared to a pig?
Moshav Zekenim: A pig is always saiated, for its food is ready (it eats even refuse). So Yitzchak told Esav - "umi'Shemanei ha'Aretz Yihyeh Moshavecha" (Bereishis 27:28). [He gave this Brachah] "Ki Tzayid b'Fiv" (25:28). It is called Chazir, for it is Chozer (returns) and eats human excrement. Pigs received all kinds of plagues; so Edom will be stricken with Tzara'as.
Pesikta Zutresa (Bereishis 26:34): A pig sticks out its hooves and says 'I am Tahor!", and all say "Gerah Lo Yigar Tamei Hu." So Edom steals and spreads money (does Chesed); about them it says "v'Chesed l'Umim Chatas." This is like Esav, who hunted and abused men's wives, and at the age of 40 he married, to make himself appear like his father, who married at 40.