An exploiter is a user, someone who takes advantage of other people or things for their own gain. Being an exploiter is selfish and unethical.
To exploit someone is to use them in a way that's wrong, like an employer who pays low wages but demands long hours. An exploiter is a person who treats others this way. Someone who runs a sweatshop that makes people work all day and night in the stifling heat for a tiny paycheck is an exploiter. Some say companies that burn a lot of oil are exploiters of the Earth.