There are, unfortunately, an infinite number of bad choices we
can make, those actions (and consequent habits) that serve to
block our path to becoming who we are supposed to become. For
Aquinas, hurting or destroying ourselves and sin are pretty much
one and the same.
We can sin in many different ways, typically by wanting something
either too much or too little. We can sin through ignorance, especially
by not knowing what we could or should know; apparently for Aquinas
cluelessness is not an excuse. We can sin from passion, because
passion gets in the way of sense, arousing "vehement and
disordered images and judgments" and creating a "disordered
desire for some temporal good that arises from disordered love
of self." We can sin from bad will, where we choose a lesser
good, knowing full well the harm it can cause, or because we are
weak and simply have become accustomed to slacking. Finally, we
can sin from pure cussedness, freely choosing to hate what is
good. This is where, from the point of view of our own happines,
we get into deadly trouble.
The most devastating sins are those by which we destroy our
natural responses to goodness. To be happy, we have to be willing
to love: to know, recognize and be fully drawn by the good. Charity
is essentially this simple act of affirmation, opening us to happiness.
It then stands to reason that the greatest blocks to happiness
are those acts by which we choose to cut ourselves off to this
possibility. It's one thing if the intent of our action is good
and we mess up through ignorance, passion, weakness or flawed
choice; it is another thing if we refuse to be drawn by good in
the first place. When this happens, we have no chance of flourishing
and of ultimate beatitudo, since love of goodness is
the only way to get there. Aquinas considers turning away from
good as fatal to our ultimate happiness (i.e., mortal) because
the very goal of our action is wrong.
Unfortunately for humans, it is very easy to screw up. Aquinas
attributes this to "the nature of our species," to our
ego-driven, flawed humanity. He finds "an inclination in
us to disordered action," which makes us succeptible to sin.
This predisposition to sin is what in the old days was known as
Original Sin, arising (interestingly) one supposes, with the advent
of the conscious ego, back when, as the Genesis story has it,
we took that bite from the fruit of the Tree of the Knowledge
of Good and Evil. I certainly wouldn't read the Genesis story
for science, but I do think the writers nailed the connection
between Original Sin and the emergence of the self-conscious ego.
And what happens to us if we do sin? The more we sin, the more
we block ourselves from true happiness. Furthermore, sin can make
us stupid; it blunts our common sense, since we usually end up
rationalizing our behavior at least to ourselves. And it weakens
whatever habits of virtue we may have developed. Sin is ultimately
bad for us because it interferes with or shuts down completely
our growth towards what we are supposed to become. It is also
obviously bad for those around us who happen to get in the way
and suffer injustice as a result.
Fortunately, besides the inherent joy in doing right, there are
two other strong forces which serve to keep our sorry selves on
the straight and narrow: Law and Grace.
"Sin turns away from our eternal good and fixes in a disordered
way on a transitory good."
(3a, 86, 4)
"The natural inclination to virtue is lessened by every
sin we commit, since all human actions set up a bias towards similar
actions in the future, and strengthening an inclination to sin
weakens the contrary inclination to virtue."
(1a2ae. 85. 1)