It is not true that believing X is true = being biased X is true. For example, when a person goes to a psychic he is biased in favor of the psychic since he wants to talk with his dead relative. This is what I mean by a bias. It is different from just believing X is true.
I don’t understand the distinction you are trying to make. Your example doesn’t help, either. A person who goes to a psychic believes that the psychic can talk with his dead relative, so in that case, believing X is true = being biased that X is true.
Perhaps you mean to use the word “bias” only when you take the belief to be false? So we can believe that 1 + 1 = 2, but we have to be biased to believe that 1 + 1 = 4? If so, it’s really not a useful distinction.
It would be irrational to judge any part of an argument in favour of intelligent design, or argument from fine tuning on the basis that the arguer is a theist.
But in the case that you have a discussion with a pathological liar, it is reasonable not to trust him.
Why is one ad hominem, and the other not?
Chris is right that both arguments are ad hominem. That is, the fact that X is a theist or that X is a pathological liar does not have any logical bearing on the truth or falsity of any particular claim or argument that X makes.
Nevertheless, as a pragmatic matter, clearly we can rationally discount the probability of statements made by pathological liars, especially if we have no independent means of verifying them, and end up having to take them on first-person authority. This is not a matter of deductive logic, but it is a matter of inductive logic. If X has been unreliable many times in the past with claims he’s made publicly, then there is a higher probability that this time X’s non-independently-verifiable claim will also be false.
An atheist will make the same sort of argument about the theist, when it comes to theological matters. He’s been wrong so many times in the past with these arguments, that as an a priori matter I’m just not going to take this one seriously, either. (Of course, the theist will do the very same move with the atheist).
But this is only an a priori move; it is something that one must do independent of the force of the particular argument or claim made. And then the force of the argument depends crucially on how much of it depends on taking X’s word as an authority. E.g., do I have to trust X’s claims of eye-witness experience with Jesus? Well, if I have independent reasons for thinking that those claims made by X are unreliable (because he tends to be overly credulous when it comes to seeing Jesus in buttered toast), then I can rationally discount them. However, if X’s argument is objective in the sense that it does not involve any claims that depend essentially on taking X’s authority on anything, (E.g., the Cosmological Argument for the existence of God is an objective argument in this sense), then the fact that X is unreliable is strictly speaking irrelevant to his argument being good or bad.