There is no abstract concept of "human nature" which exists apart from the concrete individuals, the human persons. A nature in itself cannot fall, only human persons can. As such, we cannot talk about the human nature itself being sinful. The idea of a fallen nature is a concept derived from Augustine's teachings about the original sin, which were never accepted in the East. For some reason, the western Christendom especially after Great Schism adopted Augustine's ideas as the only basis for their theology on the subject and developed a new understanding of the human condition and the fall of mankind. According to the idea, all humans inherit the sin and guilt of Adam and are in a cursed state from their birth: completely unable to seek God, personally guilty and judged to death by God, and unable to choose to do good, all due to Adam's sin which is transmitted to all of mankind.
In contrast, according to the Orthodox faith and consensus of the pre-schism Church, we live in a world which is, due to Adam's fall, subject to corruption and death, and consequently, sin. This is our inheritance from him: our mortality which predisposes us to sin. However, even if we are beset by a corrupted world in which we all fall constantly due to our human weakness and the grievous conditions that surround us, we still retain our free will. Even in our weakness, we understand that each of us is personally responsible for our actions. We do not blame God, Adam or our nature for the sins we commit, but we take responsibility and come back to God every time we fall through repentance and are lifted up by his all-encompassing love.
If the post-fall humanity itself was inherently sinful, God would be the author of sin and death. Indeed this is what many non-Orthodox Christians believe: that God himself imposed on the whole humanity a legal penalty of death due to the transgression of Adam and Eve. Thus, they make God the author of death. In contrast, the Orthodox see not God as the author of death, but Satan. Through Satan tempting man, man turned away from the source of Life, God. The natural and only possible consequence of man separating himself from the only source of life was a fall into death and corruption, through which Satan gained domination over the mankind and the rest of creation. This is exactly why Christ became man, and "shared in...humanity so that by his death he might break the power of him who holds the power of death—that is, the devil". (Heb 2:14)
So as Orthodox Christians we realize that we live in a world in which we are beset by death and corruption, and albeit our technically free will, we easily fall into sin time and time again. But this is not due to our nature itself being fallen but due our mortality which we inherited from Adam, and due to us living in conditions in which we practically cannot avoid sinning.
However, through His incarnation, death and resurrection Christ has already conquered death, glorified our humanity, and given us the means to participate in his victory over Satan and Death through the life of the Church and especially through the Holy Eucharist. We are no in complete bondage to sin, but in as much as we grow towards Christ and make space for him to live in us, we enjoy a foretaste of paradise already in this life. We do not achieve anything good from our own strength, but only through God's mercy, which we cultivate in us through repentance, prayer, and the liturgical life of the Church.
