Defamation is a false statement about another that harms reputation.
You say someone is a cheat or has a loathsome disease, it is defamation to which a defense is the truth of what is being said. You are free to express your opinions and facts you know to be true, but be careful not to say more than you know. Trademark infringement occurs the similarity of devices to identify business causes confusion in the marketplace. A business name is a mark, which may be a registered trademark, and use of the business name on the internet in a manner designed to lure potential customers away from the mark might be trademark infringement. These consideration may make a negative campaign more costly than anticipated at its outset.