To keep the corruption at bay through evolution. An interesting choice. I like it.
How about Slughorn knowing about Horcruxes? I already gave you the quote as I remember it. I’ll have to wait though to get my hands on the books and check fr the exact words.
Why would a Potion Master that enjoys socializing be aware of such devices? From where, when and from who had he found out? Did the scouting party sent by Merenre use him and his connections in return for knowledge? I find this hard to believe. Did he performed services for them, perhaps acquire ingredients for them? Also hard to believe. Did he research them? For what purpose?
So how could Slughorn had been in the know of such knowledge?
I'm not sure how Slughorn would know in canon, as knowledge of Horcruci should even be rarer than in this story. Here, Slughorn would have known lots of 'secrets' pulled from his contacts, not unlike Grindelwald and Flamel, who knew about the Brotherhood and would have guessed at their methods.
Just think, Agathon would have loved some of the parties of the 1920s in Britain, France, and esp. Weimar Germany, not to mention pre WWI Paris. The younger Slughorn would have been there too, at least in the summers