Why did it stop?

I enjoyed the movie a lot. I heard of it because I'm friends of the band who did the song in the credits so I watched it for their moral support. I love horror and despite the typical first 20 minutes I was engaged in the movie througout.

One thing I didn't understand was if the dead were coming back for vengeance in the woods and even in the city (remember when the woman said 'that makes 36' in front of the bloody drug dealer on the fence) then why did it all end when they found ONE killer, the old woman? If it is happening outside of the area, why point it out if it isnt going to be touched upon later? I understand the drug dealer was used to drive the plot with the cop, but why not use something more self contained? Does that make sense?