Miniman is right. I wouldn't cook dry kidney beans in a slow cooker without boiling them first.
"Before they are eaten, the raw bean seeds should be soaked in water for several hours and then boiled for at least ten minutes in new fresh water to degrade a toxic
compound - the lectin phytohaemagglutinin
- found in the bean which would otherwise cause severe gastric upset. This compound is present in many varieties (and in some other species of bean), but is especially concentrated in red kidney beans and white kidney beans (Cannellini beans). Although in the case of dry beans the ten minutes required to degrade the toxin is much shorter than the hours required to fully cook the beans themselves, outbreaks of poisoning have been associated with the use of slow cookers
whose low cooking temperatures may be unable to degrade the toxin."