Is being "nice" an idol in your life? Do you go along with lies because you are afraid of offending somebody? Do you smother the truth to avoid criticism?
We are called to respect the dignity of every human person, but sometimes that requires us to state a truth in charity that will certainly offend somebody. To say, "What you are doing is wrong," may offend the person who wants to be affirmed in sin, but it is certainly the charitable way when someone is teetering on the brink of hell.
Be kind to everybody -- but love people enough to speak the truth in charity even when a person will accuse you of not being "nice," in fact, of being "mean." It is anything but mean to save somebody from careening over the abyss into the everlasting fires of hell.
Pray and sacrifice for sinners. As little Jacinta said after the Fatima apparition of hell, "We must pray for poor sinners" who, as the Blessed Mother told the children, are falling into hell like snowflakes.