Abbreviate Argumentum ad Hominem to AAH and ad Hominem Abusive to AAHA.
Source: p 132, A Concise Introduction to Logic (12 Ed, 2014) by Patrick Hurley
Keep in mind that the purpose of an ad hominem argument is to discredit another person’s argument by placing its author in a bad light. Thus, for the fallacy to be committed, there must always be two arguers (at least implicitly). If it should turn out that the person being attacked is not an arguer, then the personal comments made by the attacker may well be relevant to the conclusion that is drawn. In general, personal observations are relevant to conclusions about what kind of person someone is (good, bad, stingy, trustworthy, and so forth) and whether a person has done something. [...]
[1.] Joseph Kony, leader of the Lord’s Resistance Army, has kidnapped thousands of children from villages in central Africa, murdered their parents and relatives, and forced them into military service. He has also killed thousands of elephants and sold their ivory to fund his operation. Kony is therefore a thoroughly disgusting and despicable human being.
[2.] The conclusion is not that Kony’s argument is bad but that Kony himself is bad. Because the premises give relevant support to this conclusion, the argument commits no fallacy. Another example:
[3.] Shakespeare cannot possibly have written the thirty-six plays attributed to him, because the real Shakespeare was a two-bit country businessman who barely finished the fourth grade in school and who never left the confines of his native England.
[4.] The conclusion is not that some argument of Shakespeare’s is bad but that Shakespeare did not write certain plays. Again, since the premises are relevant to this conclusion, the argument commits no ad hominem fallacy.
I see that 1 attacks Kony's actions and 3 Shakespeare's scarce education; so 1 and 3 attacks no argument whatsoever. But I do not comprehend the importance of this distinction between attacking someone and attacking this person's arguments, for the status of AAH as a fallacy.
If AAH is NOT fallacious when used to attack someone, then why does it become fallacious when used to attack this same person's arguments?