Ad Hominem

With all due respect, like so many others who post on the Internet, I think you have confused ad hominem with personalised attack, abuse, or with the disparagement of those with whom one is arguing or conversing.

An ad hominem argument commits a logical fallacy so it has to involve an invalid inference of some sort. It is committed whenever someone infers from an assumed or actual characteristic, idiosyncrasy, failing or foible in another to the conclusion that what they say is false — or even true — just because of that. Ad hominem has nothing to do with personalising a criticism as such, but with what can be 'inferred' from that personalisation alone. It has nothing to do with abuse simpliciter, either; one can infer, ad hominem, from praise just as much as from abuse. It is the inference that is ad hominem, not the personalisation, the abuse, or even the praise. Otherwise it wouldn’t be a logical fallacy. That is why it depends on inference, not abuse or praise simpliciter.

In which case, the following would be plain and simple abuse (where "NN" and "NM" stand for the name of some individual):

"NN is an idiot".

That isn't ad hominem; it’s just abuse.

This would be arguing ad hominem:

"NN is an idiot, therefore what he says is false".

So would this:

"NM is intelligent, therefore what she says is true."

[Where in both cases "what he/she says..." refers back to an argument or assertion put forward by an opponent or interlocutor in a debate, etc. I have greatly simplified the above examples so that the point is easier to see. In real life, ad hominem arguments are often far more complex.]