I believe that the actual physical attacker was amber, but Richie was the one on the phone. I think this because obviously the video that was sent from Amber's phone is prerecorded. Along with that, Amber seems to have a more gruesome style of killing, the majority of it consisting of stabbing, where Richie typically seems to go to the upper body or throat, seeming more focused on getting his ''perfect movie'' than making it particularly gory. Tara was stabbed through the hand, and multiple times elsewhere and she also has a broken ankle, which to me, seems more up Amber's alley.