sooner or later, the mentor has to step aside (or fall by the wayside) for the others to grow into the hero's journey. - JMS
It was time, possibly past time. I also think it was being used to put Snape on an arc similar to Anakin Skywalker’s—by committing this act of evil, he’ll now be trusted by the evil one and get close enough to strike at the right time. If the next book ends with Voldemort blasting Harry with lightning-curses and Snape grabbing him and tossing him into a pit, JKR needs to have her copy of Return of the Jedi taken away permanently.
I think the whole "get this secret from Slughorn" subplot was just a training exercise to show Harry that some problems can't be solved by pointing a wand at them. Dumbledore manipulating to the last.
Ginny took "breaking up for the good of humanity" awfully calmly. I figure she’s just waiting for him to get down and lonely so she can snog him at the wedding. While remaining broken up, of course—"just this once, it doesn’t mean anything." Followed by other snoggings of opportunity and then being romantic post-war.
Hmmm. PREDICTION: If they go All The Way three chapters before the end of the book Harry snuffs it in the final confrontation.
Overall I enjoyed it. Not among my top favorites, but I’ll cheerfully read it to the kids. I’ll agree that the the Tom Riddle origin story is trashing hell out of the "choices decide fate" moral. But I’ll blame that on British immersion in history rather than secret agendas.