imo
pein (pain) and madara have a plan to destroy the shinobi villages
and the blackmail Itachi to make him join them
or else they guna kill his family
which he did (with madaras help imo)
in exchange they will psare Sasuke
theres a rumor that Itachi's plan was to make him get MS
then he will teach him how to use Amaretsu
but he failed of making Sasuke kill Naruto (his closes friend)
so instead he planted one of his powerfull skill inside sasuke
before he died to protect him againts Madara and pein (pain)
anyways thats my opinion
except for the rumor