Identifying flaws in someone's past to discredit their opinion is known as:

The term "Ad Hominem" refers to a type of logical fallacy where an argument is rebutted by attacking the character, motive, or other attributes of the person making the argument, rather than addressing the substance of the argument itself. By focusing on a person's past actions or characteristics rather than the argument they present, one sidesteps the actual issue being discussed. This tactic can effectively discredit the opinion expressed, as it paints the individual as unworthy of being taken seriously, regardless of the merit of their argument.

In various contexts, using this technique often distracts from the real discussion, potentially leading to an emotional response rather than a rational evaluation of the points being made. It’s particularly important in debates and discussions, as it emphasizes the necessity of focusing on the arguments themselves, rather than the individuals making them.