It is never right for a Christian to treat others discourteously.
1 Peter 3:8-9 (NKJV) Finally, all of you be of one mind, having compassion for one another; love as brothers, be tenderhearted, be courteous; 9 not returning evil for evil or reviling for reviling, but on the contrary blessing, knowing that you were called to this, that you may inherit a blessing.
I think that covers it completely. Even when we are mistreated, even when we are provoked by bad treatment or poisonous words (evil and reviling), we never respond in kind. Instead we bless.
That certainly doesn't mean we cannot disagree with or even rebuke others when necessary. Many times that is our duty. But when we do, we must do it without neglecting courtesy. We speak the truth, but always in love (Ephesians 4:15).
Not easy! But that's God's standard.