Toss shrimp with melted butter in a bowl until well-coated.
Heat a pan over medium-high heat for a few minutes until hot. Add shrimp to the pan in a single layer, searing for a minute or until it starts to become pink around the edges, then flip and cook until shrimp are cooked through, less than a minute.
Transfer the shrimp to a plate as they finish cooking. Let them cool while you prepare the other ingredients.
Add all other ingredients to a large mixing bowl -- diced avocado, diced tomato, feta cheese, cilantro, lemon juice, olive oil, salt, and pepper -- and toss to mix.
Add shrimp and stir to mix together. Add additional salt and pepper to taste.