As would him being proficient in using them. Snape actively using them on another person whether it was the Marauders or someone else would have been a far better response than what James went with if it were true. This may be an unpopular opinion but I feel Sirius emphasised the Dark Arts angle after the fact. Because it’s not a lie but is the version of events that makes him and James look least bad.
Harry wanted to know why his father and godfather behaved in this manner. He was torn between his belief they were good men and what he witnessed. Being confronted like that it’s natural Sirius would focus on the less ugly and more easily explainable motives for why he and James did what they did. Harry did not want to hear that they were just young and stupid.
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