You may be aware that "idiot" used to have a clinical definition, and referred specifically to people with an IQ of 25 or lower. What you might not know is that many people with an IQ higher than 25 but lower than the normal range struggle to identify right or wrong in some contexts. 

Let's suppose your brother says John is bad and has threatened to hurt him, and asks you to kill John. As a person of presumably normal intelligence, we can count on you to reliably determine that killing John is not right in those circumstances. People with intellectual disabilities cannot always make that determination, and might easily decide doing what their brother asked was the right thing.

By the way, let's say you know John served time for several past murders, and you overheard a phone conversation in which he said he was going to kill your brother tonight. You have a chance to kill John and save your brother and get away with it. Is killing him right or is it wrong?