I can recognize the trend to have the Diedne not been into human sacrifices, and I think that your analysis on how games tend to treat the subject appears sound. Many games tend to address issues of culture/tradition/practices too wrapped up in unuanced or anachronistic notions of morality.
This thread being about what stories are prefered rather than a hassle over what is closest to the RAW (another otherwise interesting discussion but one found in the other sibling thread), I still think it's fair to say that the perception of the Diedne is quite influenced by how earlier editions portrayed them (vis-Ã -vis the Tremere).
I my present saga the Diedne did perform human sacrifice. Not as a constant thing, but as something that was utterly holy and revered by them. And in thread with what they believed in - and not in spite of it (as it would be to some infernalists). In this sense I have no trouble describing the Diedne as someone who did occasionally sacrifice humans and still have them be somewhat a victim of the Schism War. They were not infernalists (though the House probably like any other had a few hiding in shadows) and when I say they were victimized by the Schism War it is not to be understood as made so by deliberate conniving from any singular opponent - but due to, as described above, the simple tragedy of things spiralling out of control due to chaotic times and due to uncompromising people on both sides of the escalating conflict.
When I in the first post speak of the sacrifice the Guernici's forebears, I do not think of the human sacrifice of a Diedne during Fenicil's ritual, but on the fact that they sacrificed a bit of their ideals. They did so both when they fabricated false evidence and when they strayed from ruling by Law rather than might and deceit. In a sense the Diedne sacrificed in that ritual do become a symbol of that moral sacrifice, but the essence of it is so much more important.
And I'm fully aware of the dilemmas this will cause the day the characters -especially the one who's himself follower of a remnant Diedne line (though this is only now dawning on him)- when they realise that the people who's names they are so desperately trying to clear, actually did things that by their laterday morals are horrible acts.... Will they keep their resolve? Or will they still be able to navigate conflicting emotions? It is always much harder to defend people you do not neccesarily sympathise with.. or at least makes your question what morals are..
Now that is an interesting story to tell IMHO.