In Paul's first letter to the Corinthians, he identifies an appalling situation that is going unchecked in this congregation. It's evident from his tone that this is as serious as neglecting a fast-spreading cancer.
There is a case of flagrant immorality. This is not a hidden sin. It is out there in the open and neither the congregation nor its leaders are doing a thing about it. Paul warns that "a little leaven leavens the whole lump of dough," (5:6 NASB) meaning that it doesn't take much for an entire congregation to be infected with the virus of disobedience to God. And disobedience is costly.
Sin dishonors. It dishonors God. It injures. It injures the transgressor and others too. It opens. It opens a door of influence to dark forces. It is a dangerous thing indeed.
Therefore, addressing it is not about occupying the higher moral ground. It is simply about protecting the ground upon which the church rests, the sacred ground of salvation, where sin can be treated with grace but never with neglect. The true grace of God "teaches us to say 'no' to ungodliness" (Titus 2:12 NIV). Counterfeit grace is permissiveness. Counterfeit holiness is legalism. Neither will do.
It is not that the particular sin that is mentioned in the fifth chapter of 1 Corinthians is more lethal than others. Any sin that is willful, habitual, and unaddressed can dismantle the integrity of the congregation. Action must be taken. Paul tells the congregation that they are responsible for judging sin inside the church (and he tells them how). That judging should be done by the believers of one's community. We are not responsible for tending to the sin among unbelievers.That is God's role (See 1 Corinthians 5:12,13).
It is my experience that the Church has it backwards. We neglect the sin of our own and focus on the sin of those who are not yet part of the faith community. We seem to fixate on what is happening, for example, among the LGBTQ community. That, according to Paul, is God's business, not ours. What about the sin that so easily besets us?