Heat a large skillet over medium heat. Add the ground beef, onion, garlic, and green bell pepper. Cook until the beef is browned and the vegetables soften, breaking up the meat as it cooks.
Transfer the beef mixture to the slow cooker. Add the kidney beans, diced tomatoes, chili powder, cumin, salt, and black pepper. Stir to combine.
Cover and cook on low for 6 to 8 hours. Stir once or twice if possible during cooking.
Taste and adjust salt if needed before serving. Ladle into bowls and add shredded cheddar cheese and sour cream at the table.