When the final verse in this series is written, perhaps as soon as Sunday, on the 2012-13 Celtics playoffs, the cause of demise could be either of two entries: the Knicks defense, the Celtics’ lack of a true point guard. Go with the latter. In Games 1 and 2 of the Eastern Conference first round series against the Knicks, the Celtics made “Walking Dead” cameos after halftime. Last night, they beat the rush, ended the suspense, snuffed the drama. They stunk from the start. Hoping to feed off the enormous energy and enthusiasm that swirled through every inch of TD Garden by the presence of medical, police and fire department heroes from the Boston Marathon tragedy, the Celtics flopped. And through an offensively inept, turnover-laded 90-76 loss, they are squirming in an 0-3 hole. “Nothing worked. Not a surprise start by Jason Terry over Brandon Bass. Not the shooting. Nothing. They committed 18 turnovers. And that’s not by accident. “A huge factor,” coach Doc Rivers said of the turnovers. “We fought for a little while, kind of gave in. We got our fight back a little bit in the third quarter ... and that’s when the turnover barrage began.” The lack of a true point guard — Avery Bradley, known for defense has tried to handle the position — has strangled the Boston offense as much as the Knicks. The Celtics lost starter Rajon Rondo then backup Leandro Barbosa, both to torn ACLs, a few weeks apart in late January, early February. Bradley has tried, but isn’t a point guard. Conversely, the Knicks have ridden Raymond Felton. Rivers frequently has stressed how Felton has been “killing” the Celtics.