He is glad not to know what he's missing.
Welcoming
His first kill is during the Chuunin exam. Itachi is alone, and the desert training field is vast and empty. There are many hours left until his team meets. Meanwhile he watches the corpse of a girl, waiting.
There are thoughts, foreign. What she was like, her family? Will he be penalized? Finally he hides the corpse and says nothing. It's left behind in the sand and he is promoted.
Shisui watches the promotion from the stands, eyes vast and dark. Itachi feels cold, more than he ever did with father, but Shisui only tells him that he'll do fine.
END
It always sort of surprises me that people go "omg how did this happen" when someone like Itachi or Sasori comes along. I mean Konoha isn't the Mist with their genin-killing ways but it's still fucked up to have an eight year old committing murder on advice from his parents. And they wonder why he decides "fuck everyone"… after they shove him into the Chuunin exam at eight, yeah that's real affection right there.
I'm trying to say that being immersed in shinobi culture so unapologetically should be done cautiously… someone should put a fucking age limit down….
PS: this, specifically, is on the topic of having no mentor who would say, "no, you shouldn't kill unnecessarily".