Does money corrupt, or is money attractive to questionable people?

Both. There is a study by Armin Falk and Nora Szech (2013) that experimentally shows that markets can erode moral behavior, as participants were more likely to not save a mouse's life for money in market settings. This generally extends to the erosion of morality for monetary gain in market systems. Additionally, psychological research indicates that wealthy, successful individuals often score higher on traits associated with the "dark triad" of personality, i.e. narcissism, Machiavellianism, and psychopathy, which supports the idea that selection plays a role as well.