I'd have to agree that X the movie is a servere disappointment-as mentioned by nekova, you're not gonna fit that much info into a 2hr film. You could try the X tv series, but its a little slow, and i doubt you've the time to watch a entire series for your project. Pitty really, since the conflict kamui has to face between the oppossing agruments of the chi no ryu and ten no ryu would be exactly what you're looking for.
Since gungrave, and escaflowne have been mentioned(and are good suggestions) i'll throw out Hellsing as a maybe. [spoiler] Alucard is a monster-in every sense of the word, but he's doing the right thing by killing vampires. Or is he? They are his own kind- but in episode one alucard sums up his own personal belif regarding hunting his own kind pretty well. You also have seras interal conflict as well- her refusal to drink blood, until she sees her master feeding on a human (a "bad" one regardless) at which point she bolts back to her room to feed her self.[/spoiler]
Hope thats helpfull!