Do most Heretics know they are heretics?

Do all Heretics go to Hell?

St. Thomas (II-II:11:1) defines heresy: "a species of infidelity in men who, having professed the faith of Christ, corrupt its dogmas". "The right Christian faith consists in giving one's voluntary assent to Christ in all that truly belongs to His teaching. There are, therefore, two ways of deviating from Christianity: the one by refusing to believe in Christ Himself, which is the way of infidelity, common to Pagans and Jєωs; the other by restricting belief to certain points of Christ's doctrine selected and fashioned at pleasure, which is the way of heretics. The subject-matter of both faith and heresy is, therefore, the deposit of the faith, that is, the sum total of truths revealed in Scripture and Tradition as proposed to our belief by the Church. The believer accepts the whole deposit as proposed by the Church; the heretic accepts only such parts of it as commend themselves to his own approval. The heretical tenets may be ignorance of the true creed, erroneous judgment, imperfect apprehension and comprehension of dogmas: in none of these does the will play an appreciable part, wherefore one of the necessary conditions of sinfulness--free choice--is wanting and such heresy is merely objective, or material. On the other hand the will may freely incline the intellect to adhere to tenets declared false by the Divine teaching authority of the Church. The impelling motives are many: intellectual pride or exaggerated reliance on one's own insight; the illusions of religious zeal; the allurements of political or ecclesiastical power; the ties of material interests and personal status; and perhaps others more dishonourable. Heresy thus willed is imputable to the subject and carries with it a varying degree of guilt; it is called formal, because to the material error it adds the informative element of "freely willed".

Pertinacity, that is, obstinate adhesion to a particular tenet is required to make heresy formal. For as long as one remains willing to submit to the Church's decision he remains a Catholic Christian at heart and his wrong beliefs are only transient errors and fleeting opinions. Considering that the human intellect can assent only to truth, real or apparent, studied pertinacity — as distinct from wanton opposition — supposes a firm subjective conviction which may be sufficient to inform the conscience and create "good faith". Such firm convictions result either from circuмstances over which the heretic has no control or from intellectual delinquencies in themselves more or less voluntary and imputable. A man born and nurtured in heretical surroundings may live and die without ever having a doubt as to the truth of his creed. On the other hand a born Catholic may allow himself to drift into whirls of anti-Catholic thought from which no doctrinal authority can rescue him, and where his mind becomes incrusted with convictions, or considerations sufficiently powerful to overlay his Catholic conscience. It is not for man, but for Him who searcheth the mind and heart, to sit in judgment on the guilt which attaches to an heretical conscience.

http://newadvent.org/cathen/07256b.htm

Heresy is a sin because of its nature it is destructive of the virtue of Christian faith. Its malice is to be measured therefore by the excellence of the good gift of which it deprives the soul. Now faith is the most precious possession of man, the root of his supernatural life, the pledge of his eternal salvation. Privation of faith is therefore the greatest evil, and deliberate rejection of faith is the greatest sin. St. Thomas (II-II, Q. x, a. 3) arrives at the same conclusion thus: "All sin is an aversion from God. A sin, therefore, is the greater the more it separates man from God. But infidelity does this more than any other sin, for the infidel (unbeliever) is without the true knowledge of God: his false knowledge does not bring him help, for what he opines is not God: manifestly, then, the sin of unbelief ( infidelitas) is the greatest sin in the whole range of perversity." And he adds: "Although the Gentiles err in more things than the Jєωs, and although the Jєωs are farther removed from true faith than heretics, yet the unbelief of the Jєωs is a more grievous sin than that of the Gentiles, because they corrupt the Gospel itself after having adopted and professed the same. . . . It is a more serious sin not to perform what one has promised than not to perform what one has not promised." It cannot be pleaded in attenuation of the guilt of heresy that heretics do not deny the faith which to them appears necessary to salvation, but only such articles as they consider not to belong to the original deposit. In answer it suffices to remark that two of the most evident truths of the depositum fidei are the unity of the Church and the institution of a teaching authority to maintain that unity. That unity exists in the Catholic Church, and is preserved by the function of her teaching body: these are two facts which anyone can verify for himself. In the constitution of the Church there is no room for private judgment sorting essentials from non-essentials: any such selection disturbs the unity, and challenges the Divine authority, of the Church; it strikes at the very source of faith. The guilt of heresy is measured not so much by its subject-matter as by its formal principle, which is the same in all heresies: revolt against a Divinely constituted authority.

http://newadvent.org/cathen/07256b.htm