*NOTE* All scripture passes taken from The Scriptures 1998+ unless otherwise noted.
There are many passages in Scripture that discuss the use of speech, and how it can be used for evil. There is no doubt after reading passages like the following that cursing or swearing is a sin:
Let no corrupt word proceed out of your mouth, but what is good for necessary edification, that it may impart grace to the hearers. (Ephesians 4:29)
Calling people stupid is the same as calling them a fool and is condemned in scripture. People who talk like this usually do not know Yeshua the Messiah even if they profess to. Scripture also tells us that this behavior is devilish and is not part of righteousness.
But I say to you that whoever is wroth with his brother without a cause shall be liable to judgment. And whoever says to his brother, ‘Raka!’ shall be liable to the Sanhedrin. But whoever says, ‘You fool!’ shall be liable to fire of Gehenna. Matthew 5:22
With it we bless our Elohim and Father, and with it we curse men, who have been made in the likeness of Elohim. Out of the same mouth proceed blessing and cursing. My brothers, this should not be so. Does the fountain send forth the sweet and the bitter from the same opening? My brothers, is a fig tree able to bear olives, or a grapevine figs? So neither is a fountain able to make salt and sweet water. Who is wise and understanding among you? Let him show by his good behaviour his works in meekness of wisdom. But if you have bitter jealousy and self-seeking in your hearts, do not boast against and lie against the truth. This is not the wisdom coming down from above, but it is earthly, unspiritual, demonic. James 3:9-15
Those who are born again show a better level of communication and are more engaged in sharing ideas rather than talking about people and casting insults. A person who continually casts insults and runs people down is clearly not born again.
For He who wishes to love life and see good days, let him keep his tongue from evil, and his lips from speaking deceit, let him turn away from evil and do good, let him seek peace and pursue it. Because the eyes of יהוה are on the righteous, and his ears are open to their prayers, but the face of יהוה is against those who do evil. 1 Peter 3:10–12
Don’t mistake a rebuke for a negative comment. Messiah rebuked many spiritual leaders, yet he did not have a negative attitude even though he was revealing their sin. There is a stark difference in showing people their sin and simply running them down constantly because we enjoy arguing or we have a critical spirit. Some people might view a prophet as negative because of the message he has from the Lord. But those who are not born again are unable to discern this and they will only see a prophet as a trouble maker. The elect of Elohim know and understand the difference. Exhortation is motivated by love, however insults and put downs are motivated by an evil heart.