In polling friends as to what they think is the deepest root of all sin, I got three main answers. One was a shrug indicating no answer at all (i.e., “I dunno”). Another was to refer to Scripture: For the love of money is a root of all kinds of evils (1 Tim 6:10). I’ll discuss below why this is an inadequate answer. The third main response was that original sin (and the concupiscence that followed) is the source of all of our other sins. The only problem with that answer is that it doesn’t explain Adam and Eve’s (original) sin, nor does it explain the fall of the angels, who seem to have fallen in great numbers without original sin or concupiscence and are now demons. Therefore an even deeper root must be sought.
Referencing St. Thomas Aquinas and Fr. Garrigou-Lagrange, permit me to answer that the deepest root of all sin is inordinate self-love. From this root springs all sin, including the original sin of Adam and that of the angels. It is true that our fallen condition has intensified the problem of inordinate self-love, but the possible temptation to it was there before.
For to what else did Satan appeal when he said to Eve, and you will be like God (Gen 3:5)? And indeed, by what were Lucifer and all the other fallen angels tempted when they mysteriously rebelled and, in effect, declared their non serviam (I will not serve)? Adam and Eve as well as all the angels (though sinless and not fallen) chose to love themselves more than God. They would not love or trust God more than they loved themselves. For the angels it was a “one and you’re done” decision. For us, the drama continues, but will end with our definitive and lasting decision either to love God or to love our own self more.
The inordinate love of self is the most fundamental root of all sin. We all know its power and its pernicious quality. Even the most wonderful things we do are tainted when we do them more for personal praise and glory than for love of God and neighbor.