Genesis 6:5; 8:21 tells us that man is mentally bent toward evil. Jeremiah 17:9 tells us that man's heart is treacherous and he cannot even fully know it. Roman 7:21-25 tells us that man's foremost desire is to do bad.
"Not to limit speech to wholesome, helpful words makes the Holy Spirit feel grief because of our behavior. We are not saying that you can never say anything negative. Sometimes we are forced to talk about unpleasant things, particularly in solving problems in which people are involved. Teachers, ministers, employers, coaches, lawyers, police, and so on, all find it necessary to tell the truth about someone even if it is unpleasant. Whether you are solving a problem or not, you avoid speaking unwholesome words. Your intent is to build up, not tear down, to unify, not divide." - Max Anders