I am working under the assumption that, whatever you think of the method of obtaining Gibson Sr.'s views, you do not agree with them.
The question becomes, does the end justify the means? I think the answer depends on the situation. There was an episode of M*A*S*H where a racist Major was tricked into revealing his racist views while a Black man, posing as an Major (he was really a Captain), was waiting nearby and overheard. The racist was then tricked into resigning his commission.
I know there are big differences between the two situations, but I feel better knowing that the elder Gibson's views are public. I now know him for what he is. And while other's on this thread are content to dismiss him as a "crazy loon" or make other excuses for him or focus on how his views were obtained, I think it's important to know who these people are in our society. Get them out from under their rocks, so to speak.
...but it isn't fair even to discuss whether Jews were involved in the death of Christ, two thousand years ago?
I am at a bit of a loss as to where you saw this. Can you provide some sort of evidence that "it's not fair to discuss it."
In one case, there is guilt by association with an ancestor; in the other, it is not even allowed to discuss the actions of ancestors.
How does that work?
Well, in the first case the relationship is not strictly "ancestral", but that of a father and son. The assumption, right or wrong, is that a father's beliefs and behaviors influences the son. Since we all learn from our parents to some extent I don't think this is a stretch. The influence is somewhat greater than one's ancestors who passed away over 2000 years ago, wouldn't you agree?
Be that as it may, there is little to discuss regarding the Jewish "ancestors" because there is no single point of reference. I assume you believe in the truth of your Bible, correct? But I, as a Jew, must reject the entire document. Simply put, my belief system doesn't believe the events portrayed in your bible ever occurred. Hence, what is there to discuss? Your beliefs? How can I possibly comment intelligently on that?
Bear in mind that I haven't said one word about Mel Gibson or his film. Does Mel share his father's views? On the contrary, he has repudiated the views while still embracing the man. It is difficult for some to understand this. I, on the other hand, can understand it. Also, having no evidence that he has lied, I have to take Mel at his word, that he does not share the father's views about the Holocaust.
As to his movie, I am tickled pink that Mel made the movie and it is receiving all this attention. Putting aside my loathing of Foxman and his liberal "Jewish" cohorts, I enjoy seeing Christians celebrate their faith. Christianity is under attack and has had some rather nasty affronts in the last few years...."The Last Temptation of Christ" and the exhibit in the Brooklyn Museum a few years ago...it's good to see a movie which is positive about Christianity for a change.