Rosenberg was doing a little drugs deal on the side without Roys knowledge, he roped in Joey and Anthony and thought - "why pay for the drugs when I can just kill them and take them for free".
He may have got away with it if he hadnt used the alias "Chris DeMeo".
This was traced back to Roy (obviously) and as Chris was his guy he had to sort it out.
The Gambino's werent gonna allow Roy to start a war for someone as lowly as Chris Rosenberg and so Nino ordered Roy to whack Chris and be done with it