I haven't seen the movie for ages, but I remember the book. The point is that their 80's yuppie society is so superficial that EITHER :
He has really been killing people but they're all so interchangeable and care so little about each other that no one really notices.
OR
He's just been imagining the whole thing, and the reason he's thought his 'victims' have gone missing is because they're so interchangeable.
Yes, the book is better.
*puts on scarf and beret, puffs knowingly on pipe*