Why do some people often criticize historical figures for their alleged atrocities?

Two ends of a spectrum here.

First:

“Statute of limitations” effects, whereby people nominally adult but still in school do things that were culturally acceptable at the time, but in later life their past comes up and hammers them. Not saying much about last summer’s Kavanaugh witches-pot; rather, two of the three top politicians in Virginia AND the top Republican, fourth in the line of succession should the first three all go away, did things as young men that nobody objected to at the time.

But today, after centuries of prejudice so endemic we can’t even see its active remnants today, the boil is bursting and the cumulative mess inside that boil now comes out with such force that old men’s actions thirty-five years ago, as “young adults” who were still in school, are judged by today’s sensitivities and hung around these men’s necks, full weight and wet as fresh paint . When a boil bursts, watch out!

Second:

In a somewhat related manner, people doing what was socially acceptable in their era wind up at the wrong end of a telescope, again being judged as though their acts were committed in today’s social context. A big example is Thomas Jefferson’s offspring via Sally Hemmings. There’s no way to whether one of his sons fathered any of her children – but the timing argues against it – they weren’t there at the right times. So we judge Thomas Jefferson for flagrant delicto’s which were very common at the time, since slaves weren’t considered as other than some wholly deficient subset of human beings. Comparing the rights of a caucasian woman to the rights of a black woman, in that era, was like comparing an apple to a – fly speck? Outrageous today, but a social ‘given’ then.

In other words when we hold public figures from bygone eras up to the harsh light of this day’s moral opprobrium, the real critique should go toward that bygone era and not the figures who were alive then. People to criticize are those which violate their own era’s concept of decent behavior. Violating this era’s metric is notable, but no modern practitioner of blue-nose prudery has any right to carp.