So we have the humour with which da Ponte leavens a potentially extremely dark situation, and which makes Giovanni someone vainly trying to recapture the sexual success of his younger days. Then we have, overlaid upon this superficially funny structure, the powerful music of Mozart. It's that sublime soundtrack which has done the most to convince audiences (and directors) that there must be more to Giovanni than meets the eye - why then set the story of his decline and fall to music as wonderful as that? If this were some sordid rapist and nothing more, why hold us rapt with such majesty? Authors have heard that music and read into it more than perhaps Da Ponte and Mozart intended - remember E.T.A. Hoffmann's piece of fanfiction Don Juan, in which Donna Anna is heard confessing that she fell in love with her alpha male of a rapist? (Urgh.) But Hoffmann heard in that music the note of striving and longing to which Romantics thrill, and Don Giovanni became for them another archetype of the man that seeks and will not yield, not even to the voice of Heaven compelling his obedience or else his destruction.
It's the music. Furthermore, it's the music which condemns Don Ottavio as weak and effeminate. One music critic - alas, I forget who - pointed out that whilst the other men in the opera are given lively solo pieces set in a bass or baritone register, Ottavio is given two slow, soaring and undeniably feminine arias in a sweet tenor range. Put those pieces side by side with something like "Fin' chal dal vino" and which comes across as the more masculine? Furthermore, remember that "Il mio tesoro" is sung at a time when he should be preparing to avenge his beloved: listen to those long, langorous notes, especially the one which is a test of any tenor's breath control, and you might get the impression that he's more interested in how he sounds than what he does :-) Personally, I think Ottavio gave himself away with that "Ohime, respiro!" after Anna stated she fought him off: not "Thank Heavens you were all right!" but instead a very camp expression of relief that Anna hadn't been violated. He couldn't possibly marry a rape victim, now could he? Phew, he breathes again. And the modern woman is not impressed with him. Consider also that Anna refuses to give him his happy ending: why would she do that, if she didn't have serious doubts about him as a husband? (Please don't tell me it's her grief for her father; it's my belief that those suffering such a loss tend to cleave more strongly to those loved ones they have left for support. Had she loved Ottavio, I think she would have married him in an attempt to "compensate" for the pain left by her recent bereavement.)
So, there's my argument for why Giovanni is frequently presented as the antihero we hate ourselves for loving. It's a flimsy structure of belief, to take this failed seducer and make him into a towering hubristic Romantic hero, but I've seen worse. Like, as you say, the idea that Draco Malfoy is going to redeem himself overnight :-) Such interpretations appeal to the heart and not the head, and we know that Mozart's music for DG strikes the heart more quickly and cleanly than any other operatic score in existence.
Devil's Advocate argument, Part II
It's the music. Furthermore, it's the music which condemns Don Ottavio as weak and effeminate. One music critic - alas, I forget who - pointed out that whilst the other men in the opera are given lively solo pieces set in a bass or baritone register, Ottavio is given two slow, soaring and undeniably feminine arias in a sweet tenor range. Put those pieces side by side with something like "Fin' chal dal vino" and which comes across as the more masculine? Furthermore, remember that "Il mio tesoro" is sung at a time when he should be preparing to avenge his beloved: listen to those long, langorous notes, especially the one which is a test of any tenor's breath control, and you might get the impression that he's more interested in how he sounds than what he does :-) Personally, I think Ottavio gave himself away with that "Ohime, respiro!" after Anna stated she fought him off: not "Thank Heavens you were all right!" but instead a very camp expression of relief that Anna hadn't been violated. He couldn't possibly marry a rape victim, now could he? Phew, he breathes again. And the modern woman is not impressed with him. Consider also that Anna refuses to give him his happy ending: why would she do that, if she didn't have serious doubts about him as a husband? (Please don't tell me it's her grief for her father; it's my belief that those suffering such a loss tend to cleave more strongly to those loved ones they have left for support. Had she loved Ottavio, I think she would have married him in an attempt to "compensate" for the pain left by her recent bereavement.)
So, there's my argument for why Giovanni is frequently presented as the antihero we hate ourselves for loving. It's a flimsy structure of belief, to take this failed seducer and make him into a towering hubristic Romantic hero, but I've seen worse. Like, as you say, the idea that Draco Malfoy is going to redeem himself overnight :-) Such interpretations appeal to the heart and not the head, and we know that Mozart's music for DG strikes the heart more quickly and cleanly than any other operatic score in existence.