Factual error: In the scene when Will is opening the drawer of films from the Leeds home, there is clearly a copy of Mrs. Doubtfire in the left column of tapes. How can that be? Red Dragon is clearly set "several years" after 1980, as the caption says, but before the 1991 Silence of the Lambs, but "Mrs. Doubtfire" came out in 1993.
Factual error: In the scene where Will Graham is flying from DC to Baltimore to return the note to Lecter's cell, we see a panoramic view of DC and the Potomac River. We see him fly over the FDR Monument (located on the Cherry Tree Walk on the Tidal Basin near the national mall). Although that area was authorized a Park on September 5, 1959, the monument itself wasn't constructed until 1990 and was dedicated on May 2, 1997. It wouldn't have been there for him to fly over.
Factual error: When the FBI scrambles the helicopter to return the letter to Lecter's cell in Balimore, the camera pans across a scenic view of Washington. You see the famous sites - the Washington Monument, Lincoln Memorial and the US Holocaust Memorial Museum in the bottom right of the screen. Trouble is the Museum opened in 1993 and the movie is set before 1991.
Factual error: The dates of the two murders do not match any year of the 1980s. 25th of February occured on Saturday in 1984 and 1989, but the moon was not full on that particular day in 84 or 89.
Factual error: Towards the middle of the movie there is a quick shot of a van driving down a road. To the right of the van on the side of the road is a solar cell that is powering a Department of Transportation device. This is the type of solar cell you see for the construction arrow signs on the side of the road telling you that you must merge. These weren't around in the 80's.
Factual error: When Will sees the bedroom covered in blood, the blood appears bright and fresh, despite the fact it had been sitting for several days and should have looked dark and coagulated.
Factual error: Sitting on the island shore discussing the murder, the table on which they place the piece of paper is a Ikea table, wrong for the period. (00:13:29)
Answer: His choices had to do with the layout of peoples' property. At his job, he studied customers' family video tapes that contained scenes of their homes and yards. He looked for seclusion around the properties, easy-access back entrances, whether there was a family dog that would bark, and so on.
But what was his reason for killing them? Was it because he saw a happy family and he was angry because he never had one or because he saw a life that he would never have?
He chose the houses that had big backyards. During the Edward Norton/Lecter interaction, Lector says something about how blood looks in the moonlight.