No. NO! Seriously, it is not safe. Read on to learn why dry red kidney beans should never be slow-cooked.
Red kidney beans contain a potent toxin called phytohaemagglutinin which can cause severe gastrointestinal symptoms, including nausea, vomiting, and diarrhea. Fortunately, this toxin is deactivated by the heat of boiling. So boiled kidney beans, and this includes canned beans, are perfectly safe. But slow cooking uses lower temperatures than boiling, which is 212 degrees Fahrenheit. The actual temperature reached in a slow cooker varies by model and the setting used, but it is always below boiling-this is the very definition of slow cooking! Thus, the toxin in kidney beans will not be deactivated, at least not fully, by slow cooking.
You can safely cook kidney beans in a slow cooker if you soak them overnight and then boil them for 10 minutes before putting in the slow cooker, but it hardly seems worth the trouble! And as I already mentioned, canned kidney beans are perfectly safe.
By the way, other beans (for example cannelini and turtle beans) also contain phytohaemagglutinin but in much lower concentrations than red kidney beans. I have not seen any warnings about cooking those in a slow cooker.
Funny, you should bring this up. A retired Ph.D. organic chemist, I knew this, but confess that knowing and remembering are sometimes two different things. I have a package of red kidney beans that I have planned to cook in the near future. I was just waiting to do so until the food we already have cooked and ready in the frig is down to a reasonable level.
In my case, this is a timely reminder and is much appreciated.