A schemer is a person who, in secret, makes clever plans that are not honest, so they can get what they want. The word already judges the person: you would not call a friend a schemer unless you were angry or warning someone. Writers and newspapers use it for people who work behind other people’s backs. Though actually, the shorter word scheme is milder in British English — a “pension scheme” or a “colour scheme” has no dishonesty in it at all. The person-word schemer almost always keeps the dark meaning. (In computing, a Schemer can also mean someone who writes in the language Scheme. That is not this sense.)