I have always thought it was a little naïve of Harry to assume that Sirius and Lupin, who have, after all, fought a war, had not killed anyone; but come to think of it they always use "stupefy" and "expelliarmus" and the like later on, and "avid kedavra" is forbidden -- are Lupin and Sirius really planning on using a forbidden curse in the Shrieking Shack? I found it strange to place so much emphasis on not wanting them to become killers (yes, I am jaded by television), but it makes sense in the wider context of horcruxes and how the series develops.