Busirane is probably the most unlikeable villain in all of the Faerie Queene, and trust us, he's got a lot of competition. Busirane's very physical violation and torture of Amoret, in contrast to the largely psychological violence caused by other villains, makes Busirane pretty easy to hate on.
Busirane's capture of Amoret (whose named has associations with love), is the culmination of his symbolic role as a perversion of real love. Incapable of understanding love's symbolic qualities, he literally takes Amoret's heart out her body and enacts a kind of metaphoric rape by penetrating her chest with a steel pen. And just in case you weren't disguised enough by his interpretation of love, his house is covered in images of rape and the main source of entertainment is a performance of Amoret's rape. Ick.
Since the steel rod penetrating Amoret is also described as a pen (punning on 'pen' as both a writing instrument and a place of enclosure), Busirane can also be seen as a perverse foil to the poet, who "pens" women all the time in writing about them. This suggests that Spenser might be critiquing the sexualizing of women that male poets routinely engage in, and perhaps even critiquing himself, since his poem certainly engages in plenty of that.