Are you talking about "The Zodiac" from 2005:
http://www.imdb.com/title/tt0371739/
Or "Zodiac" from 2007?
I have seen Zodiac (2007) and I liked certain parts, but it was looong!
I thought the part KY mentioned was creepy, but when he stabs that couple.... that was fucked up. It was pretty much the most realistic murder I've ever seen in a movie.
Also, the part where the main character is in that guys basement and you here someone upstairs and the lights go out... FUCK THAT SHIT! That was so creepy.