We saw that Severus tried to make the detentions more unpleasant for Harry to make them sink in more, and that he hinted (mendaciously) that they should be continued the following year. But I think he saw that his fundamental strategy wasn’t working, and that he did come up with something else.

Now, it wasn’t actually a stupid strategy, given Harry’s reaction to SWM. Harry had always brushed aside Snape’s criticisms of James and Sirius, but when he saw an example of their actual behavior it appalled him—not least for forcing him to admit (temporarily, at least) that Snape had been right about them. So the detention notices were supposed to demonstrate to Harry that SWM had not been a one-off—that had been the Marauders’ standard M.O. Moreover, it was important that Harry see that Snape was not the only victim, since Harry might decide (especially, er, after a certain incident) that Snape deserved what he got from them.

But it didn’t work. First, reading about James and Sirius hexing other kids for no reason didn’t have the same visceral impact as watching them do so. It gave Harry an unpleasant “jolt”  each time, but no more than that.

Secondly, Harry could always tell himself that even if they were arrogant bullies at fifteen, they both died courageously. Heroically. So Harry the proto-hero might tell himself they were still good role models.