And this is exactly how I myself feel about the mafia genre as a whole 
As I said before, I won't scream at people who like the genre or anything like that. But yep, portraying mobsters as the heroes (note: "heroes" as in "good guys, role models, people to admire", not as in "protagonists") is something that irks me to no end.
Which isn't to say that people shouldn't write about mobsters being involved in a love story at all: as awful as they are, these guys are human too and they do have relationships in real life (with some of them being quite famous, too), so it'd make no sense for me to say that nope, you should never ever write about a mobster falling in love. Also, as long as proper research is done and the work of fiction shows the dark side of the mafia, I'm more than okay with it. I probably still won't read it, because generally speaking heist stories are not a genre I have an interested in, but you do you.
I also agree that one way to make people to "root" for the criminal would be to make their opponent a much worse person... however, I find that kind of thing often leads to the whole "the clan only kills people of other clans, no innocents involved ever" which kind of defeats the idea of showing the true dark side of the mafia. As I said in the other thread, mafia has absolutely no qualms killing innocents, and writing a story where "yeah, they're bad, but they only kill people who are worse than them" does fit into what I'd call "romanticizing" the issue.
Same goes for portraying mobsters as the Robin Hood of the story: in my country especially, that kind of attitude is not fiction. It's reality. The mafia presents itself as the only thing that can "save" you from a government that just won't care. They pretend to help you, they give you "jobs", money to live... and then, if you don't do exactly what they say, they kill you and your entire family. Making you believe that they're the good guys and that they're some kind of rebels going against the rules to help people is exactly one of the most used tactics of the mafia over here to recruit people. So, once again, not exactly the kind of stuff I'd like to read about, unless it's being handled with the necessary care (read: the true consequences of the Robin Hood attitude are being shown and they're not just left at the "yeah, they're rebels, but good rebels" stage).
As I said, however, these are just my views as someone who lives in a city/country plagued by the mafia. The thing that irks me about it is that, unlike with most horror movies where the murderer/monster/evil guy is CLEARLY evil and you're rarely going to see them being depicted by its author as "omg, so sexy!!!1!", unlike war movies where the war IS usually depicted as something that kills people, no matter on which side they are, heck, unlike with Death Note which truly makes you question what the line between good and evil is... with the mafia genre, the kind of attitude I see the most on the Internet is "GUNS, SEXY SUITS AND ROMANTIC ADVENTURES, OMG MAFIA IS SO COOL!!!" which... really rubs me the wrong way.
I don't know how many of you are from the US, but to put things in context, it'd be similar to having someone creating a whole romance genre about... school shooters. Y'know, stories about how these "poor kids" are just "misunderstood", how they're "actually good guys" who got tired of a "society who constantly bullies them", so they decide to create an alliance to "get rid of all them popular kids" as a "revenge". It does happen in real life, am I right? A lot of school shooters are outcasts who could never fit into society. However, now imagine the school shooter in this kind of stories being constantly depicted as a romantic hero who "just wants to make the world a better place" because "society failed him". The kids who get shot, instead, are always depicted as the "bad guys" who "deserved it" and nobody will cry for them. And then, imagine having an entire fandom of people who are not from your country and who never witnessed the horror of a school shooting going around screaming stuff like "omg, school shooters are SO SEXY, I wish there was one in my school!", "yes, plz, gimme a sexy school shooter from the US!" and "shooters are so heroic, omg!!!! -swoon-"
If this example sounded utterly disturbing to you, then I guess you can understand how I (and many other Italians) feel about the mafia genre.