I believe those who insult others do this to make themselves feel more superior. By putting someone else down, you falsely elevate your position (but only in your mind; to the outside world, you look even more inferior).
Insulting is a sign of immaturity, and immaturity is frowned upon in this society.
The act of insulting others is particularly common among people who are deeply insecure. If you're not feeling good about yourself, you try to bring other people down to your level.
Christ-like leadership never uses insults against God's precious people. "But whoso shall offend one of these little ones which believe in me, it were better for him that a millstone were hanged about his neck, and that he were drowned in the depth of the sea." Matthew 18:6.
"But if you give them a hard time, bullying or taking advantage of their simple trust, you'll soon wish you hadn't. You'd be better off dropped in the middle of the lake with a millstone around your neck. Doom to the world for giving these God-believing children a hard time! Hard times are inevitable, but you don't have to make it worse — and it's doomsday to you if you do." (Matthew 18:6-7 MSG)