A Clockwork Orange - "Singing in the Rain"

I think Stanley Kubrik used the song "Singing in the Rain" for the shock value and to contrast the more innocent version by Gene Kelly.  It shows the extreme contrast from dancing around in the rain to the violence in A Clockwork Orange.  It is also a reflection of what was happening in society at the time.  The Vietman War was happening when the movie was made and released.  The indiscriminate violence of war was reflected in the random violence inflicted upon the couple.  The singing added a perverse element to their rampage.