In a large bowl, combine 1/2 cup ketchup, 1 1/2 tablespoons prepared horseradish, the juice of 1/4 lemon and a pinch of crushed red pepper. Reserve 1/4 cup of the cocktail sauce. Add 1 dozen peeled and deveined tail-on jumbo shrimp to the bowl and toss to coat. Cover and refrigerate for 4 hours or overnight. Preheat a grill to medium-high. Thread the shrimp at the head and tail ends onto each of twelve 6-inch skewers. Brush both sides of each shrimp with extra-virgin olive oil. Grill for 3 minutes, flip and grill for 1 minute more. Brush with the reserved cocktail sauce.