Red Dead Redemption 2 keeps surprising us. You will have to find three map locations.
All serial killer map locations in RDR 2:
You found the first part of the map east of the Braithwaite estate. Focus on the last letter S in the inscription Scarlett Meadows. A scroll will stick out of the head from the back of a huge tree.
You will find the second piece of the map under the railway bridge to the south-east of Valentine (see screenshot). The head with the scroll will stick up from the wooden support of the bridge.
The last piece of the map you found on the stone south of the Wallace Station. The exact location is shown in the screenshot below. After that you need to put all three fragments and get a map.
There will be a new additional task "American Dreams" will appear on your map to the south-west of Valentine. Upon arrival, break open the door to the basement and examine the knife in the far room.
See a cutscene and grab the serial killer. Tie him up and take him to the local sheriff in Valentine. When he tries to close the criminal in the cell, he will pounce on him. Shoot a maniac and get $ 20 in reward.