Stir together ketchup, vinegar, Worcestershire sauce, paprika, salt, pepper, chili powder, and garlic in a small bowl. Reserve 1/2 cup of the mixture and store in the refrigerator.
In a slow cooker, add the pork roast, the rest of the ketchup mixture, onions, and water. Set to low and cook 8-12 hours until the meat is tender and falling apart.
Use two forks to shred the pork then move to a serving dish.
Serve on rolls with a dollop of the reserved ketchup mixture. Or for low-carb, skip the roll and place a serving on a plate with a side of vegetables. Use the reserved ketchup mixture as a sauce.