I've seen quite a lot of arguments against judging historical figures by modern standards, most representatively by the principle of moral relativism. Is there any argument for it though? The only example I've got so far is the notion of moral obligation, in which the impact of a historical being's action is evaluated, and resorted to a contemporary being that is believed to have inherited the obligation(e.g. the contemporary Australian government apologising to Aboriginal Australians on behalf of the Australian government in the 20th century).
Standards are never universal or consistent enough to be anachronistic as such. In America, we have records of prominent founders acknowledging the wrong of slavery, even guilt and dread over this, and indeed abolitionism was part of the ethical milieu thereafter through the Civil War.
Kant later, and las Casas earlier (and much more intensively), are examples of critics and condemnation of European imperialism/colonialism vs. native Americans.
Theories of just wars stretch back well over a millennium.
And on top of all this, what do you think the victims thought? Were they generally like, "According to the dominant moral theories of my day, my murder or enslavement or rape is acceptable, so tally-ho!" No, even if damaged to the point of submission, there is no need for us to assume that the victims really agreed with the atrocities inflicted on them, at least not due to mentally healthful reasoning and attitudes.
Present apologies for previous generations' misdeeds
A present-day Australian apologising for the treatment of Aborigines by his forbears? You probe the moral position that supports or explains this practice.
A historical person - a forbear in this case - can only have acted on the moral beliefs they had. Only such beliefs can have provided their motivation for action. In terms of moral responsibility and conscience, such a person cannot be judged with any point or justification by later standards since they cannot have been motivated by such standards. We cannot blame them for failing to act towards Aborigines on moral standards to which they had no access.
This is perfectly consistent with putting blame aside and judging that none the less by our own standards a historical person - the forbear - acted wrongly. They did an action or carried out a policy, motivated as they were by their own moral beliefs, which by our standards is wrong even though they did what they thought was right or permissible.
This seems to me a coherent position: the forbear (or more realistically a group of forbears, most likely spread across generations) acted wrongly by our standards, and so we apologise, or express moral regret, because we have benefited by what the forbear - or group of forbears - did and because we enjoy advantages which still disbenefit the Aborigines.
On a theoretical note, since you mention 'moral relativism'...
This viewpoint, supporting the appropriateness of apology or the expression of moral regret, has no particular connection with relativism. An objective or cognitive morality is not static - or at least our knowledge of it may not be so. It may be understood differently at different times; a historical person may have a less perfect, a less developed, understanding of its requirements than someone living at a later period. All that is relative is, in respect of moral responsibility and conscience, the relation of a historical person's motivation to their moral beliefs. 'Relative' here has no connexion with 'relativism' if this protean term signifies that no action is right, obligatory, neutral or wrong except relative to the non-truth apt and contingent norms of a community, society or culture.