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.
Also Mickey and Manny were in business with Nucky so if Mickey was going round telling people he killed Manny over business i think Richard brought him to Nucky to basically say ok tell Nucky what youve done, cos obviously Richard knows its not true. So when Mickey denies it Richard says i know he didnt do it cos i did, because he killed Angela. He wanted people to know, or Nucky at least that he killed Manny because of Angela, someone he cared about maybe even loved who was innocent was killed. So he didnt want an oaf like Doyle using the murder to his advantage in business deals, maybe even in Richards eyes [or eye] it was disrespecting Angela. He wanted him to know it was him that did it and why, for Angelas sake.
'As far back as i can remember, i always wanted to be a gangster'
