2. Mental Incompetent.
3. Stupid Person
A person who is a complete idiot, is not just doing 1 idiotic act, which would only make them an idiot. To be a complete idiot you must perform a series of stupid actions.
Uncle: Look at that man, he just finished playing 'ding-dong-ditch' on his own house.
Neighbor: Oh, don't worry about Dale, he is just a complete idiot.