Heat a large skillet over medium-high heat, then add the beef and cook for 4 to 5 minutes, breaking it into small pieces as it browns. The meat should lose its raw color and start taking on browned edges as fat renders into the pan.

Stir in the diced onion and cook for 4 to 5 minutes until softened, then add the garlic, cumin powder, chili powder, garlic powder, dried oregano, salt, and black pepper for 30 seconds, stirring the whole time. The onions should look tender and glossy, and the spices should smell robust and fragrant instead of dusty.

Transfer the beef mixture to a 6 Qt slow cooker, then add the black beans, kidney beans, diced tomatoes with their juice, diced tomatoes and green chilis with their juice, and tomato sauce. The slow cooker should look full and evenly mixed, with the meat and beans fully coated in the red tomato base.

Cover and cook on high for 3 to 4 hours or on low for 6 to 8 hours, then taste and season more if needed before serving warm. By the end, the chili should be thick, deeply red, and unified, with the beef fully cooked and the flavors mellowed together; ground beef should reach 160 °F for safe serving, as outlined here on ground beef cooking and leftovers safety.