Oh, you did not just go there. I'm a bloody history buff.
By whose standards do we judge? Cao Cao did some terrible things, but he also created a more meritocratic and fairer justice system for the land. A lot of this just comes down to them living in another age. Most historical people of great importance did both terrible and good things.
Also, Zhao Yun and Guan Yu were historically good guys if you don't count the fact that they were warriors who killed for their lord. And if that makes them "real-life villains"- what does that make most of the Soul Calibur cast?
While I don't disagree that the historical Liu Bei was a jackass, it has nothing to do with the point.
It was literally a statement invented by the author living in the 14th century, to make Liu Bei seem more virtuous. so the statement is actually from a time period close to SCs own, rather than ancient- and to add to that, partly propaganda for the Ming Empire; you know, the very place Xianghua comes from in canon. So it reflected that time rather than any ancient time.
The point is still that sworn brothers were more treasured than wives or lovers(or even your own children) during the actual era of SC. I don't disagree that it's sexist. Kyam going from sworn brother to lover is a downgrade plotwise, because avenging a sworn brother fits the era better by far(and it also lets us get away from the more western view that romantic love trumps all other love, even familial one). We already have an avenger of love- Setsuka.
