Cook Pasta: Boil spaghetti in salted water until al dente (about 12 minutes). Drain and set aside.
Make Sauce: Whisk together mayonnaise, sweet chili sauce, lime juice, and Sriracha in a bowl.
Cook Shrimp: Melt butter in a large pot, add shrimp, and cook for 5 minutes. Stir in garlic and cook for another minute.
Combine Ingredients: Add the cooked spaghetti and sauce, tossing to coat. Let cook for 2 minutes until hot.
Let it Rest: Turn off the heat and let sit for 5 minutes to allow the sauce to thicken.
Garnish & Serve: Divide onto plates, garnish with chopped scallions, and enjoy!