Taking care of the dark matters and ordering the massacre of the Uchiha are different. He tried to foster peace thru negotiation, Danzo did what he did behind Hiruzen's back.
Btw, not chapter 628.
So, you think that Itachi would have been able to carry out the Uchiha Clan Massacre without the approval of Hiruzen?
Yes, it was 627. Sorry, a typo.
Danzo pushed for the Massacre, like I said. Hiruzen gave the nod when his negotiations failed. So, yes, he tried to do it another way, but when negotiations failed he just went ahead and had them all killed. Dafuq?
The only one who can assign missions is the Hokage. Hiruzen gave Itachi the mission of massacring the Uchihas. Itachi reported to him that he had completed the mission successfully, requested him to take care of his brother, then left.