The insanity trial of admitted killer Christopher A. Gribble begins Monday with the selection of jurors who will decide whether Gribble was legally insane when he and Steven Spader slaughtered a Mont Vernon mother and maimed her daughter during a random 2009 home invasion. His trial in Hillsborough County Superior Court likely will differ from that of his machete-wielding accomplice Spader, found guilty by a jury Nov. 9 of all six counts of first-degree murder, attempted murder and related charges. Gribble, a Brookline Eagle Scout, admitted he stabbed and slashed local nurse Kimberly L. Cates, 42, to death with a...