Jonathan.Link:
Well, there is nothing canonical about House Diedne, except what has been written in the ArM5 book. So, any opinions are likely to vary wildly.
There has been a bit about the Diedne spread around the supplements in this edition. Here's a thread from 2007 that gathers a good number of the statements about the house together
It is the stated intention of the line editor and the authors to leave the question of Diedne guilt unanswered in the published material. I believe that they've done a fine job of it.
Yet the topic has generated a good deal of heat here. Specifically Boxer has provided a strong voice for painting the Tremere as vile evildoers. Also Tim among others has brought in powerful arguments demonstrating that the fifth edition texts do not provide a strong case that the accusations of human sacrifice…