I don't know how things are gonna worse than naught(zero). or is it possible?
Sure, it's possible. Suppose I offer you your choice of either being attacked by a bear, or not being attacked by a bear. Would you agree that the latter option is "nothing", and the former option is "worse than nothing"?
Or, more relevantly — think of a mass murderer. The victims' families would not notice if someone said nothing at all about the murderer, but you can imagine that they would probably be offended if someone described him only as "a bit of a jerk".
In the sentence you quote, which comes from Edgar Allen Poe's "The Literary Life of Thingum Bob, Esq.", the eponymous narrator is saying that the insults make the editor of the Gad-Fly sound less bad than he is, and that they are such egregious understatements as to be worse than saying nothing about him.
But it's worth keeping in mind that Mr. Bob is an utterly ridiculous character, and we're not at all meant to agree with him.