He told Itachi he was going after Madara next because he suspected Itachi couldn't solo the clan. He needed help.
Itachi was forced into it by the Leaf, but also he still continued to protect Sasuke.
Madara(Tobi) helped, but he didn't have a reason(Not that we know of yet). He wanted some Sharingan(s).
He forgives Itachi, but what Madara(Tobi).
P.S. I'm saying Madara(Tobi) because I don't believe Sasuke knows about Edo Madara.
Itachi was forced into it by the Leaf, but also he still continued to protect Sasuke.
Madara(Tobi) helped, but he didn't have a reason(Not that we know of yet). He wanted some Sharingan(s).
He forgives Itachi, but what Madara(Tobi).
P.S. I'm saying Madara(Tobi) because I don't believe Sasuke knows about Edo Madara.