To me, as I see it from the writings of Blessed Metropolitan Philaret of New York, a person who professes beliefs different from Orthodoxy is a heterodox. He is just honestly following what he has been taught. A heretic, however, seems to me to be someone who preaches something against the truth, who is blinded by pride and will not listen to reason, nor let others alone who confess the truth. A heretic can be heterodox, or may have been Orthodox at one time. An apostate is someone who leaves Orthodoxy for another faith. The terms are not, in and of themselves, pejorative, but can certainly be employed in such a way.
Agree 100%. Historically, the term "heresy" meant simply "a choice" or "a separate opinion," and it was not necessarily used in a negative way; St. Paul even encourages people to have these "heresies" (Greek ""αἱρέσεις," "eresis") or separate opinions" "so that those who are approved may be recognized among you" (δεῖ γὰρ καὶ αἱρέσεις
ἐν ὑμῖν εἶναι, ἵνα οἱ δόκιμοι φανεροὶ γένωνται ἐν ὑμῖν) (1 Cor. 11:19). But now, when the Church has Her dogmatics that passed the Seven Ecumenical Councils, someone who stubbornly preaches something that is blatantly against these teachings of the Church is a "heretic" in a negative sense. For example, Jehovah's Witnesses or Oneness Pentecostals, who reject the full divinity of Christ and/or the Trinity, are definitely heretics. Monophysites who reject the full humanity of Christ are also, clearly, heretics.
As for Protestants, I, too, think that it would not be fair to label them all "heretics" because they may hold very different beliefs. For example, C.S. Lewis was an Episcopalian (still a Protestant denomination, although many Episcopalians would not agree), but I don't think there are any heresies in his books like "Mere Christianity."