I think it is the law that you believe you are under.
Protestants seem to think that they will be judged by the torah/Law, which demands perfection. James saying that any infraction against the Law makes you guilty of breaking the Law (or, as some believe, makes you guilty of breaking every single law) becomes the epicenter of the earthquake in a Protestant's life - the realization that in God's eyes, they are considered guilty. Everything follows from this need for perfection. This is what makes faith an absolute necessity, so as to cover your imperfection. On this site, almost every post from a Protestant is aimed at pointing out our imperfection. We are to follow God's commands, but do you? And I mean always, at every moment? How well are you doing with that? Good luck with that. The Protestant wants to bring people to a realization of their imperfection - and this includes themselves and all believers. Because the torah/Law requires perfection, as James said.
On the other hand, the Catholic believes that the reborn person who now is living in Christ has passed from the torah/Law (if they were ever under it, assuming they were Jewish) and is now under the law of Christ, which is a new covenant, and also known as "divine law." This law does not demand perfection, but that your heart be in the right place. It is a merciful law, a law of freedom, the opposite of the torah/Law that demands perfection, as James himself said.
What you get then, ironically, is that Protestants complain that Catholics are too assuming of God's mercy. They seem to accept their sins and not think them so bad, because they are the sins of a child who makes mistakes and will be forgiven by God's merciful loving-kindness. They have confidence in God's mercy because they understand mercy themselves - they believe God uses the same measure of mercy as they themselves use with others, as Jesus taught. The Protestant generally tries to puncture this confidence in God's mercy by trying to get the Catholic to the realization that all their sins are equally life-threatening, because they need to be perfect under the torah/Law, and their only assurance lies in their faith.