First of all, not all people of Fire Nation descent are necessarily anti-Aang; keep in mind that Zuko and Iroh were good friends of his and after the Hundred Year War he tried hard to cultivate good relations with the defeated Fire Nation.

Shyamalan's film seems to have just been genuinely, sincerely bad (historians of the era still have parties where they watch it and point out the inaccuracies; the grad students at my department had one such party shortly before I started but they said that it was too depressing to repeat). Whether or not his great-grandfather was Fire Nation or not (I won't be bothered to check if the rumors are true), he had no cultural connection to the Fire Nation and has never shown any sign of Fire Nation sympathy. I think that the issues Turnshroud mentions come down more to terrible writing than malice- for example, if he's so sympathetic to the Fire Nation, why wouldn't he bother to portray firebending accurately?

(on a side note, I'm a little bummed that TLA put a nail in the coffin of historical epic films about the Hundred Year's War, but honestly there were some really quite good animated serials by Varrick's animover division that do it plenty of justice (partly because Varrick wasn't really involved in them)).

No, but it's suspected that he has certain sympathies for the Fire Nation. Why else would the earthbenders be imprisoned in an earthen prison, needing the Avatar to make them realize that they have earth all around them? Why would he make it sofirebenders can't bend their own fire? If you ask me, it's evident that M Night Shyamalan is a Fire nation apologist. It's just so obvious that he wanted to downplay the strengths of the Fire Nation to make them appear weaker than they actually were.

It would also explain why Aang is so whiny, and Katara is close to useless in his so-called 'movie.'