Jude, starting at verse 8 thru the end, is speaking of those who slyly speak against others who are obviously approved by God and their direction and decisions that are obviously being blessed.
Instead of cooperatively supporting those approved by God, like Cain they assault their brothers, even if only verbally, and then when the damage is done, take an aloof attitude, in effect arrogantly stating "am I my brother's keeper?"
Like Balaam, whom Jude said was looking for a reward, such men seek authority and recognition in the congregation as a selfish reward to be grasped, at the risk and expense of wholesome unity and spirituality of everyone affected.
Like Korah, who challenged Moses' rightful leadership, those who promote their own ways as better and oppose direction given by those whom God is blessing, demonstrate that they are actually working against God Himself. Indeed they are "despising authority." (vS8) It would be a different situation if these turned apostate and went off to form their group. Instead, they seek to subvert the congregation by remaining in it.
In all these cases, those who act like that would have done themselves a favor if they had only conducted themselves like Jesus who, verse 9, left the matter in God's hands. Yet in that instance Jesus would have been 100% correct in setting Satan straight. In contrast, none of our brothers are Satan, so how much more careful we need to be when we feel some indignation over a perceived wrong. Regardless of how wrong or right our view is, arrogance, stubbornness, and insulting words are not the answer.
Now this is not to say that we cannot express our honest viewpoint just not in arrogance or at the expense of our brother's dignity. Nor should it be something that we talk around to others, spreading dissension in the congregation. If we truly have a problem with a person, we should first approach them alone. But to spread negative talk is exactly what Jude referred to as "rocks under the water" (vs.12). Such rocks in the ocean could completely tear a ship apart. We don't want our words, attitudes, or actions to tear the congregation apart.
Although in context verse 12 was talking about prideful divisive forces in the congregation, in grander application this would apply to anything that could tear apart a congregation, especially cunning and premeditated motives.
To reduce The effect our imperfect speech can have on others, remember the carpenters saying: "measure twice, cut once." Indeed, think carefully and then measure out your words slowly. Address the issue, but cut short before insulting your brother or becoming a divisive, corrosive agent in the congregation.