Judging by the behavior of David toward Shaw in the prologue clip and the apparent nature of his character and relationship to Shaw as depicted in A:C, something extremely drastic had to occur that we still don't know about yet.
I hope it is something juicy that is focused on primarily in ADF's prequel novel...because you can't show something to audiences, and then do a 180 and show them something completely opposite without some exposition about the events that transpired in the gap that caused the change. Unless you're Damon Lindelof, that is, and his career seems to be doing so well for it (sarcasm mixed with contempt).