Have you seen the movie Zodiac? In that movie, the killer is nice and slow, taunting people in the newspaper.
Stretch the adventure out. Make the PCs feel like the Raven is watching them constantly, and is taunting them through the Inquisitive.
You can do this by having an adventure going on during the Raven's escapades. Just lengthen the time between his kills, and have it a grim reminder that pops up every other turn. This way, you can have it like something looming in the background. It'll distract the PCs - if they pursue the Raven, then their current mission might falter, or the plot of the Other adventure drags them away.