Richard didn't like that Mickey Doyle was taking credit for killing Manny. It went against his sense of "honor", especially that it was being done by a simpleton (Doyle).
I think Nucky understood this.
And ofcourse he wouldn't be Nucky if he didn't try to manipulate him into becoming his personal assassin. He needs all the guns he can get in the inescapable war with Rosetti.
"It was between the brothers Kay -- I had nothing to do with it."
