Heat a skillet over medium heat and add the ground beef. Cook until browned, breaking it up with a spoon as it cooks, then drain any excess fat.
Transfer the browned beef to the slow cooker. Add the diced onion, minced garlic, and diced green bell pepper.
Stir in the drained kidney beans and the can of diced tomatoes with their juice.
Add the chili powder, ground cumin, salt, and black pepper. Stir everything until the spices are evenly distributed.
Cover and cook on low for 6 to 8 hours or on high for 3 to 4 hours until the vegetables are soft and the flavors have blended.
Stir the chili before serving.