I put it to you that in matters of religion and political philosophy – character matters. It matters a lot. It matters so much that the validity of a philosophy or religion should rise or fall based on the character of the person who founded or created it. This is simply because when we consider a philosophy or religion (especially ones making universal claims), then the character of the proponents very much matter. It matters because the effects of such beliefs go beyond a single person or group and depending on their appeal, could easily affect many people for good or for ill.
Yet whenever one goes on to point out the miserable reality of Ayn Rand’s objectivist existence, the fact that Karl Marx relied on plenty of capital and was no personal friend of the working class; it is likely that this will be dismissed as ad hominem. It is not at first glance unreasonable to respond this way. And I do still think that in a formal debate setting, that the ideas rather than the person should be critiqued. Arguments in general should also ideally be considered on their own when they can be.
When we are talking about belief systems though, the life and character of the prophets, founders, philosophers and thinkers I would argue are absolutely fair game and should be considered. Questions should be asked. Did they live by the values of their philosophy? If not, did they at least make an honest effort and were they upfront with their failures? If not, why not? What were the results in their own life of living their way if they actually did? These all matter and I think are absolutely relevant when considering belief systems.
I have been disappointed myself by philosophies and philosophers who I admired when I saw how they worked or lived so I state all this as someone who has been let down. And though I certainly think that ideas and arguments do still have to be considered on their own merits, I would observe that more often than not, the apple has fallen far away from the tree.
I’d say that from the first time you witness weakness in a parent or authority figure, it deals a blow to your worldview and all of us being human, that is inevitable. It is however, of a much higher order of magnitude for the one who set the standard in the first place to fail to live up to it. That makes the person fair game for personal attack.