Since the end of 6A, Tony was phasing out Christopher and Paulie from his inner circle.
Chris yeah, but Paulie?
Chris was an addict. An addict can be trusted to be unreliable and Chris had risen too high up to be able to be unreliable. Tony killed him here because of his close relationship with him years earlier when he promoted Chris while overlooking his numerous faults and then it got to a point where they weren't as close and Chris couldn't be saved by Tony's favoritism this time.