Sin is a kind of irrational force that leads a person to her demise while convincing her that she is doing the right thing. For example, Jesus tells the religious authorities the parable of the tenants, which ends with the warning that the vineyard owner will kill the tenants and give the vineyard to someone else. They perceive that he was telling the parable about them, which means that they understand his threat: if they kill him, they will suffer punishment from God. Yet when they are confronted with the truth and the danger of their own situation, what do they do? Rather than repent and ask for guidance, they are hardened in their sinfulness and determine that they are going to kill Jesus! Sin made them to throw themselves headlong into the destruction about which Christ warned them.
This irrational force of sin is devastating, however, because it convinces us that there's nothing wrong with what we're doing. On the contrary, we are doing the right thing! The religious authorities were convinced that they were doing the right thing by killing Jesus, even if he showed them through the parable that they were opposing God himself. This is why we ought to have a healthy skepticism of ourselves and constantly check ourselves by the teaching of Christ and the apostles, lest we be deceiving ourselves.
Beyond irrationality, however, though perhaps in some way related to it, sin is characterized by a kind of childish desire to escape consequences for our actions. Sinful persons want to enjoy themselves and get pleasure out of life, but without suffering any consequences for their actions and without exercising actual responsible agency. For example, consider people who want the pleasures of sexual activity without the responsibility of parenthood that naturally accompanies this. These persons either use contraceptives or force an abortion to escape the burden of caring for a child, but they still want to engage in sex despite its natural result. In this manner, they want to escape responsible agency: they want the benefits without any of the costs.
Consider another example: a person who commits serious crimes, seemingly feels no remorse over what he has done so long as he is free, but as soon as he is caught and is faced with inevitable punishment, cries and wails and laments his fate pathetically—not out of remorse or conscience, but only because he was caught and will now suffer. Such a person is overwhelmed by the power of sin and wants to escape responsible agency out of his slavery to the pursuit of pleasure.
There are ordinary cases, too. How many people do I know who swear they will turn their lives around, but only later, after their youth has passed! No, they tell me, now they want to enjoy themselves, so long as they are young and able to do so easily; repentance is for later in life, when circumstances will constrain them to be more responsible. This, too, is an evasion of agency: their actions merit punishment and in some sense they recognize this, but they defer repentance till later in life, when the consequences will be minimal and continuing their pursuit of pleasure would prove too burdensome or impossible. These persons refuse to exercise real agency and instead leave themselves at the mercy of their desires, impulses, and circumstances, holding out an irrational hope fortuitously to avoid all negative consequences they deserve.
There is no avoiding the consequences of our actions, however. Catherine of Siena makes the point that no one can escape from God's hands, and at the end of the day, we will all have to give an account for the life we lived. Either we will be found by God repentant, in which case we meet with his mercy, or we will be found in sin, and so meet with justice. But there is no avoiding God, and so there is no avoiding responsibility and accountability for our actions. God has made us free, such that we have the responsibility of developing our characters either towards good or evil. Sin tries to make us lazy and to abdicate this responsibility, which will only make our characters vicious and ungodly in the end. Thus a person can be damned effectively by refusing to exercise responsible agency and taking control of her actions.
There may be an interesting eschatological argument here. Annihilationism might be considered a weak doctrine on the grounds that a sinful person does not have to live with the consequences of her actions, if she is eventually annihilated. Actual suffering for sin is limited, even though annihilation lasts forever, whereas her offense in turning her back against goodness itself (God) is infinite in gravity. If sinners are therefore eventually annihilated, their abdication of their agency turns out not to be a particularly grave thing; the offense is relatively small. Rather it is as if they have merely opted out of playing a game that was avoidable. What's to stop a sinner from deciding to go on in sin and eventually be annihilated, since all actual suffering is limited in duration and nothingness offers no pains or discomforts?