If saying “Thou fool!” puts people in danger of hell fire, which Jesus said (Matt. 5:22), what about your other “little” sins?
What about “looking on a woman (or a picture of a woman or man) to lust after her?” (Matt. 5:28) Christ calls that adultery and God made that one of his basic ten “shalt nots.” (See Exodus 20, whole chapter.)
Are you guilty of such things as cursing people or mental adultery after you became a Christian? Most of us are.
We think we cannot help it, but we can if we have been born again.