I have come to the conclusion (after several days' shock and pondering) that Sev is STILL a good guy. See, he HAD to promise Narcissa that he would help Draco because if he didn't Wormtail would report it to Voldie and Bella would blow up at him and Draco would have be in even more danger. So he HAD to promise, and if he didn't do Draco's job when it became obvious that Draco couldn't, then he, Severus, would die, and later Draco, Lucius, and Narcissa would die at Voldemort's hand. So, therefore, Sevvie is still good.