The concept of "death" is a mystery, just like "time" - as St Augustine says: "when I do not ponder about it, I think I know what time is, but when I ponder and try to express, I am exasperated, for time does not exist: past - is already not; future - is not yet; now - is more fleeting than an eyewink, but "time" cannot be an eyewink". The similar mystery is death, for we think it is, but what it is? And when we are afraid of death, what are we afraid of? I may be afraid of fire, for it burns when it touches me, but is death something like a fire? Does it hurt when it touches me? - Impossible, for, as Epicurus says, "when I am, death is not, when death is, I am not, so we never meet". If so, then what are we afraid of? Of being deprived of our habitual surrounding - friends, loved ones, things, attachments - and being left alone? But alone where? In which new surrounding? We do not know and that frightens us? But will there still be "we" when death comes? Will we perceive anything and think of anything? Shall we be conscious after death? If not, then what are we afraid of? Of a total annihilation?
But does not the very idea of total annihilation make the meaning of life, goodness, virtue etc. obsolete? Because, it is totally unjust for the universe to treat both good and bad persons absolutely equally, that is to say, handing both eventual absolute annihilation, and moreover, granting to bad ones sometimes better fortune in the earthly life, which is the only life. But how can something unjust be also good? Thus, if the universe and its order is unjust, then it is not good. But we read that God is good. Then God is not unjust. If so, then, if total annihilation still is the destiny of all, then we can conclude that the universe does something unjust to us, while God is unable to do justice, even if He necessarily would tend to do so, for He is good, and as good, also just. But such a God cannot be a biblical God, for the biblical God is almighty and cannot be weaker than the universe which He created.
Thus, if the universe is subject to the good God, then it is false that the universe can annihilate us entirely, for God then would be tarnished by an idea that He left the evil-doing of the universe unattended in such a fundamental thing. Therefore, from a biblical perspective, total annihilation cannot exist and we have retribution for our deeds on the earthly life, and if so, then we must retain our consciousness also after death, but if so, then we must also retain life after death, for it is impossible for something not living to have consciousness, and if so then we have something in us that is not reduced to body but outlives its death. That something can be called "soul", "mind", "inner man", "self", "core of personality" - you name it! Both Christianity and other great religions and philosophies (some of them at least) tell us that the main concern is to preserve this undying core in us in a healthy state for the dimension of life, in which even body can be exempt. Thus, I think, there can be two sorts of fear of physical death, the first foolish and unphilosophical, the second wise and philosophical: the foolish fear is to fear total annihilation and deprivation of our life's attachments (or the second without the first, with a prospect of retaining consciousness in a totally alien and undesirable environment, which is even more foolish, I will not go to prove this self-evident thing), whereas the wise fear is to fear a damage of our core-personality, that does not die together with the body, but is to come to the presence of good God, and unless it is in a healthy state, this presence to God and of God can be perceived as unpleasant and undesirable, like, for instance, for a husband who cheated upon his wife, the presence of his loving wife can be perceived as totally unpleasant and shameful.
Now, let us discuss the first, foolish fear. This fear, the fear of sheer fact of physical death can be beaten by a greater fear. A soldier can risk his life if the alternative would be a public shame, which he would dread more. Or, a fear of loosing a political freedom and coming under a bondage of an enemy can also be a motivator for overcoming the fear of a physical death (and such people are praised by Aristotle as μεγαλοψυχοί /magnanimous/). Or, by fear of danger for life and safety of loved ones, as a loving father would rather die for his children's safety. Those instances of overcoming a fear of physical death by some other fear is called "courage".
But there is still another courage, of which Plato speaks: a courage to oppose one's evil inclinations and harmful passions. For instance, a married soldier courageous enough to sacrifice his life for the freedom of his city, but still not courageous enough to overcome his womanising urges and give up his frequent peccadilloes committed outside his marital union with wife, is not courageous in this second, Plato's sense, and even if he sacrifice his life for his city, his soul is not healed from this passion, but retains it even after the death of body; for, as to Plato, soul does not die together with a body and the health of soul and its post-body-mortem destiny depends on its being healthy, that is being liberated from harmful passions. You can be healthy as a trout, bodily, but still utterly ill in your soul, if you do not fight inner tyranny of passions through philosophy. But a true philosopher, with a healthy soul that is in love with the eternal, invisible good realities, should, in principle, not be afraid of the physical death, because he will know that physical death only will make his communion with the eternal and bliss-providing realities even more intensive and unhindered.
Christianity is close to Plato's intuitions, in this sense, for according to Christianity "the cause/sting of death is sin" (1 Cor. 15:56), but Christ came in order to deliver all humanity from iniquities, in order to destroy the power of sin and rescue humanity from its drive (cf. Romans 7:24), consequently, to destroy also the death which is grounded on the sin. But this death cannot be a physical death, for when we are "dead by sin" physically we are still alive. James says that the committed sin brings death to the one who committed it (James 1:15), but such "dead" people could be healthy as trouts! On the contrary, saints are more afraid of sin that brings this metaphoric death than of the physical death and if given a dramatic choice, choose the latter in order to avoid the sin and the metaphoric death. Because they are philosophical, aristocrats of Spirit, not plebeians of Spirit (such plebeians can be even kings and greatest earthly aristocrats), not foolish, but wise, knowing that there is incomparably greater danger in damage of our inner, body-surviving core of personality than even in the bodily sufferings and the bodily death, for "our light and momentary troubles, [and a physical death], are achieving for us an eternal glory that far outweighs them all" (2 Cor 4:17).
Thus, to answer your question: the Hebrews 2:14-15 can be understood in two ways:
Devil has power over humans, because humans know that only with Moses' Law (or Plato's philosophy) they cannot overcome the power of sin, and therefore also of metaphoric death of our inner core, i.e. the life deprived of the inner presence of Christ and the Holy Spirit, that is the result of the sin. Thus, they fear death and in order not to make its dominion too overwhelming, check sin through precepts of Law (or philosophical practice, if outside Jewish religion), but this does not liberate them, but rather indicates that they are under the fear of this metaphoric death that they cannot in fact beat.
Devil has power over humans, who fear physical death more than the metaphoric death of the inner core, of soul, who fear more physical death than sin. Those humans do not understand in their utter foolishness and mind-cloggedness that by accepting Christ in their lives they will be able to destroy the dominion of sin and the metaphoric death, and that this is incomparably more important and desirable than continuation of earthly life in the state of metaphoric death.
Moreover, Christ, in difference from Plato, gives even greater liberation, for He promises with unfailing promise that not only our metaphoric death will be abolished through Him (which is the pivotal thing), but also our physical death, for He has a power and sovereign authority of resurrecting dead bodies also (cf. John 10:18), like He showed this authority by resurrecting Lazarus who was dead for four days, or resurrecting His own dead body after three days.