It is possible to have a huge knowledge of theology as if one's head was a library of theology books, to preach against sin, to really be serious about sin, to hate sin and to be a "walking Bible" but it is also possible to really have another motive other than loving people. It is not possible to love someone and never tell them the truth but it is possible to tell the truth without loving the person. It is like you can never love your child without chastising the child but it is possible to chastise the child without love for the child.
It is really true that Christians are called to rebuke sin and to speak against it without compromise. Ephesians 5:11 says, "And have no fellowship with the unfruitful works of darkness, but rather reprove them." but is the rebuking against sin done in love for people, loving them enough to tell them the truth or just for personal reasons? It is possible to rebuke sinful people because you do not want them to burn in Hell and it is possible to rebuke sinful people because you just want to look holy and just. It is possible to be speaking against sin while having the sins of the Pharisees that says, "Lord, I am thankful I am not like the other people." Even when Christians are saved, it is possible to still have that "Thank God I am not like the others." attitude which even if this person is aware that a changed life is by God's grace, it is still very possible to spite a person who is lost and self-righteous because of the ongoing struggle with sin.
The Christian's bigger problem is not the habit of living like the rest of the world. Instead, it can be the fact that they can also feel the "I'm holier than you." attitude in them never mind that grace is undeserved, that good works come out of the believer because of an attitude of gratitude and that a changed life is by a grace of God. Even if Christians cannot live like the rest of the world but it is possible that there are times their sins might be related to the sin of self-righteousess. It can also spring out when they are preaching the Gospel, they are getting so rejected that they might already be praying for fire and brimstone to hit that person who does not listen, they get judgmental towards the unsaved saying that the person will never get saved as a result of their frustrations.
One can even go ahead and preach against sin, preach Hell and judgment and so on but what if the motive was not out of love but just to spite people? Even if the sermon was done so great, maybe you will see people coming to get saved because they were pricked but what if the person had the same problem as Jonah? What if the preacher was expecting that while he was preaching the powerful sermon against sin hoping those hoodlums would die of a heart attack instead of wishing those crooks would get saved before it is too late? All of that would have been a waste of an hour or two of preaching if it was not done out of love.