I don't think Itachi cut off his emotions. He simply did so even with his emotions. However all Hashirama was saying is that Itachi was a great man, it doesn't have to do with whose stronger.
it was an insult. because back then the definition of shinobi was a ruthless tool= murderer. thereofre hashirama basically just said "dude your bro is a douche, more so than me"
you mean doomed thousands of people to slaughter and unleashed a second madara on the world. why do you think hashirama is currently trying to undo all the hate that itachi gave to sasuke? hes trying to save konoha from itachis sociopathic torturing methods.