An ad hominem argument is a kind of logical fallacy that tries to argue from supposed facts about a person to a repudiation or endorsement of something specific that the person said or did (e.g., so-and-so did bad thing x, so they must be wrong about y).
Sometimes such facts are indirectly relevant to a more specific evaluation, but still there is no necessary logical connection — only at best a sort of Humean accidental association, or what a lawyer would call circumstantial evidence that does not constitute proof. In exceptional cases, careful consideration might legitimately weigh very strong circumstantial evidence enough to sway a decision, but this ought to be rare, because even strong circumstantial evidence is not proof.
Influenced by both Nietzsche and Hume, Gilles Deleuze tried to argue against the current that questions about who are more important than questions about what. I don’t think this succeeded.
The Postmodern Peripatetic 