I'm reminded of Harry's reaction to Eb being the Blackstaff. For (now, really obvious) reasons, Eb's training of Harry emphasized the intertwined sacredness of human life and the First Law of Magic. Harry really believed that Eb lived and breathed this ethos, and then he found out that his grandfather had a body count of lots^lots...and had used magic to hit that high score. Magic, to Harry, as taught by Eb, is the five elements bound by human will. In order to do magic, you must Believe In and Will your goal, much more than any casual physical action. ...And Eb had no problem killing mortal minions of the Red Court at Chichen Itza in job lots with a wave of the Blackstaff. This was pretty emotionally crushing to Harry.
Of course, later, his grandfather murdered Harry in order to reach and murder his other grandson...using magic.