It pushed him over the line

This line appears near the end of the movie, “Psycho”. The psychiatrist is describing Norman’s dminished mental state and when his mother had an affair “it pushed him over the line”. This is a mashup of “push (one) over the edge” (cause one to commit someone to doing something they had been considering for a long time) and “cross/over the line” (to cross some threshold into unacceptable behavior). Great catch from Vicki Kovacs.