Frankenvorta said:
The reason Anakin needs to be heroic is that he needs to do what any fictional character must: convince us that he's worth telling a story about, or hearing a story about.
By that logic, all fictional characters must be heroic. Obviously, Star Wars fails spectacularly in that regard, as does any other fictional work.
They don't have to be heroic, but they do have to have some quality or combo of qualities that makes them worth bothering with. I'll trot thru some effectively written and acted villains to give examples:
Darth Vader - Strong, decisive, powerful, self-confident, determined, insightful, intelligent. He has his own vision about how things should be and doesn't let anything stand in his way. He also has a great dry sense of humor, if a tad on the fiendish side. And he's a devoted father, in his own way. If PT Anakin had lived up to at least some of this description, it would have been a huge improvement and also made it much easier to believe that Anakin and Vader are essentially the same person, which the way the PT left it, is simply impossible.
Gul Dukat - Highly intelligent, determined, resourceful, cultured, charming, adaptable, great sense of humor, shows loyalty to friends and allies.
Scorpius - Highly intelligent, determined, resourceful, intimidatingly powerful, fair-minded (oddly enough). When you understand the full scope of his story, you start to wonder whether he's even a "villain" at all.
Gaius Baltar - The other guys are all "strong characters" so in fairness, I'll include Baltar as an example of why being a contemptable, snivelling weakling is not fatal for a character, as long as there are some counterbalancing qualities such as: intelligence, charm, great sense of humor, admirable survival skills, and strategic cunning (when he isn't being self-defeating, that is).