Aren't all sins equally offensive to God? After all, James 2:10 says, "For whoever keeps the whole law but fails in one point has become guilty of all of it."
Don't ignore 1 John 5:17: "All wrongdoing is sin, but there is sin which is not mortal." Everyone sins and falls short of the glory of God--there is no disputing that. But why would Paul tell Christians in Rome to keep the faith--"otherwise you too will be cut off" (Rom 11:22)--unless he feared for their salvation? There are other times when the apostle indicates the necessity of remaining with Christ lest salvation be lost (1 Cor 9:27, Phil 2:12). Yet salvation is not lost by every sin; as James says, "We all stumble in many ways" (Jas 3:2).
It stands to reason that the Catholic Church would teach that some human failings are worse than others. Man-made law reflects this insight: Governments do not hang jay-walkers. As it is with human law, so it is with divine law. Minor sins are called "venial," and serious sins are called "mortal" because they involve a massive rejection of God's law and cause the spiritual death of the soul.
What James means when he says that whoever fails on one point of the law is guilty of breaking all of it is not that all humans are equally guilty if they sin once--then there would be no difference in the levels of punishment people would receive, yet Jesus says there will be (Lk 12:47-48; cf. Mt 10:15, 11:22-24). What James means is that anyone who breaks one point of the law is guilty of breaking the law itself, of breaking it as an entity. To give an analogy, anyone who breaks one part of a plate is guilty of breaking the plate. He may not have broken every part of it--smashed it into pieces--but he is guilty of breaking the plate as a whole.
In the same way, a person who breaks one law has broken the law as a whole; he has become a lawbreaker, which is James's point, as is clear from the next verse: "For he who said, `Do not commit adultery,' said also, `Do not kill.' If you do not commit adultery but do kill, you have become a transgressor of the law" (Jas 2:11). This means all of us need mercy and therefore need to be merciful (Jas 2:12-13).