Rurouni Kenshin (the manga, not the movie)

Finished Rurouni Kenshin (the manga). Liked it well enough.

The one thing I don't quite understand: when the story is in the present (in 1878) Battosai is treated very much like Kenshin's alter (evil merciless murderer) ego that is very-very different from actual Kenshin, and can wake up under sufficient stress, and has to be controlled - basically the whole thing is treated as some split-personality disorder.

However, when we see actual Battosai in the flashbacks, he is very much teenage Kenshin: very nice when he is not doing his job as an assassin, not particularly bloodthirsty, and quite idealistic. He certainly doesn't seem any more different from Kenshin than a person at 18 would normally be different from themselves at 28.

If this is some general allegory of people's relationship with their teenage selves, I don't quite get it.