It doesn't matter if
Patrick Bateman killed people in the book/movie
American Psycho. This concept is fairly important to the central point of the entire story- Everyone is so self-centered and makes so many assumptions based on the constructed, artificial society in which they live that they couldn't believe
Bateman a murderer regardless of whether he killed or not.
This is similar to the more heavy-handed approach that the film Gattaca takes- people in the advanced society could not believe that "one of their elite" could be a fake, a killer.
Merely because the point is made more subtely in American Psycho doesn't mean it isn't there. Bateman tries to confess at one point, and consistently puts himself at risk of discovery. But this is only half a commentary on his character. The other half is a commentary on his society that essentially ignores everything but the self. They wouldn't believe it even if he is a killer.