Tarnishing James's Image
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.
Thirdly, the lesson is undermined. By almost all of Harry’s house. Not directly by Minerva this time, and not by Hermione (although “But you can’t call that Sectumsempra spell good, Ginny-look where it’s landed him!” [emphasis mine] shows, er, an interesting moral sensibility). But including, quite explicitly, Ginny-if Severus had read in Harry’s eyes the warm glow he felt remembering Ginny calling Sectumsempra “something good up his sleeve,” he’d have good reason to want to keep Potter away from the girl as much as possible. As a bad influence. As for the others, the Gryffs under Minerva have apparently NO moral compass at all-they explicitly regard Harry’s missing Quidditch as more important than his almost killing someone with what Harry himself considers “horrific dark magic.” And once assured that his punishment hasn’t lost them the Quidditch Cup, they are more interested in his romance than in his criminal behavior.
Furthermore, the lesson that Harry should not model himself on James had already been undermined before it started, because for six years Dumbledore has been encouraging Harry to do just that. Dumbledore himself has been reinforcing the agreeable-to-Harry belief that James was worthy of emulation. Who are you going to believe, Harry, Dumbledore or your lying eyes? Much less, Snape’s assurances backed up by a paper trail….
But finally, the lesson that Harry shouldn’t emulate James HAD to fail, because James wasn’t the role model Harry was actually following sixth year. James (or selected-James-traits) was the role model Dumbledore had selected for Harry, yes, and the one Harry believed he might be following. But Harry had not started entertaining himself by hexing random passersby “just because he could.” He hadn’t turned his friends into a baying pack, ganging up to hurt and humiliate someone “ for existing.” No, Harry was using jinxes on, and curses marked “for enemies.” Getting them back. While sneaking around after the worst, trying to prove that he should be expelled. And anything Harry did was justified because his targets deserved it. Well, almost anything. He didn’t mean to go too far-
The “guide and friend” Harry was explicitly following was the Half-Blood Prince. And even when the Prince led him into inadvertent almost-murder, Harry didn’t want to give up his guidance.
Well. There is an extremely easy way to persuade Harry that he doesn’t want to take the Prince as his role model, after all. And doing the reveal after Dumbledore’s murder will make the shock to the boy-and hopefully the effect-all the greater.
I mean, it was obvious even on the first reading-okay, for me it was on the second reading-that Snape was trying to teach Harry in the “Flight of the Prince” scene. However hampered he may have been by his Death Eater audience.
As Severus couldn’t have counted on that opportunity, I expect he left an author’s note in the book itself, so Harry would get the shock when he went back to the Room for his beloved cheat-sheet.