Heat vegetable oil in a heavy skillet over medium-high heat. Cook and stir chicken until browned and no longer pink in the center, about 8 minutes per side. Transfer chicken to a slow cooker, draining any excess oil. Add tomatoes, chicken stock, kidney beans, onion, jalapeno pepper, and chili powder to slow cooker.
Cook on Low for 6 to 8 hours or on High for 4 hours.