Why isn't it safe to cook kidney beans in a slow cooker?
Kidney beans contain a toxin called Phytohaemagglutinin, also known as Kidney Bean Lectin, that, if the beans are not cooked properly, can cause extreme nausea, cramps, vomiting and diarrhea. The symptoms typically start within 1 to 3 hours of ingestion and can last for up to 4 hours, or longer. Only a few raw beans are enough to bring on symptoms.
The good news is that the toxin is destroyed by heat. The bad news is that not all slow cookers reach a high enough temperature to do that. In fact, under-cooking the beans can actually increase the risk from the toxin.
For safety, raw kidney beans need to be soaked overnight, drained and then cooked in boiling water (212°F or 100°C) for at least ½ hour. No references that I found specify if longer cooking is needed at higher altitudes, but it won't hurt to extend the time.
After the required cooking time, the drained beans are safe to a use in slow cooker recipes. Note that commercially canned beans have already been cooked sufficiently to destroy the toxin and can be used in slow cooker or other recipes.