There are many people who commit evil actions, eg murder or rape people. Are those people harassed by demons or are they mentally ill? What is the Orthodox view?
Yes, we are tempted by demons, and they rejoice when we sin. But the question of sin is deeper than that.
To sin means to miss the mark, as an archer might miss his target. The target God set for us when He created Adam and Eve is to be living icons of God. We are created to be images (Greek: icons) of our Creator. We are commanded to grow into God's likeness.
Sin derailed this process, which we call theosis. Sin alienated us from God, Who is the Source of all life, and so we began to die. And with death came fear, panic, spiritual blindness, confusion.
Rather than living in harmony with God we began to clutch at life as a drowning man clutches at straws. The fear of death drives us to spiritual insanity. From our conception we live in fear. We fear heat, cold, hunger, thirst, loneliness. As we grow, so do our fears. Ultimately we fear death as we rush to meet it.
This fear drives us to grasp at every passing pleasure, no matter how self destructive it might be. This fear is the source of anger, greed, gluttony.
Death makes us fear. Fear makes us miss our mark. And so we sin.
So you see that Orthodoxy does not treat sin as the infraction of rules, but as a terminal disease.
When we confess our sins we do so not to accept a legal retribution, but to receive spiritual healing from our self inflicted wounds. God is our Physician, the Healer of our souls and bodies.
God loves us. He punishes us to get our attention and correct our actions - not to collect some kind of fine of every rule broken.
Forgiveness is about healing. It's about restoring us to spiritual health so that we may grow into God's likeness. It is to prepare us to pass through death with courage and clarity, and to prepare us to face the resurrection with joy.
You might want to read this essay which addresses your question very well: