OP: This might help you and your DM clarify things - the following is a direct excerpt from the EbCS on Alignment (pg 8)

"Alignments are relative gauges of a character or creature’s viewpoint, and not absolute barometers of affiliation and action; nothing is exactly as it seems. Alignments are blurred, so that it’s possible to encounter an evil silver dragon or a good vampire."

Which IMO trumps any other source book on the matter. Basically if you read the intro bit in the Eberron Campaign Setting you'll see that they have taken some liberty with the general rules "bending" them as they see fit for this particular campaign setting.