In Metro 2033 the screen would flash when you did something humane, such as giving money to the poor or strumming a guitar, and you would get a special (non-canon) ending if your humanity was high enough. So far I've had a few flashes in Metro Last Light. The actions that seemed to trigger them were strumming a guitar (the game is nothing if not original), listening to the armory officer (why?), killing a Nazi as he was getting up from his cot and killing a whimpering Nazi that called his mother "easy" (not a direct quote).
While there is absolutely nothing wrong with killing Nazis, whimpering or not, the flash I saw after killing him seemed slightly different, and I, strangely, haven't seen a flash every time I killed a Nazi.
Is the game now tracking actions it considers inhumane as well as humane? Is there an effect on gameplay or story depending on your humanity score? If the flashes I was getting are indeed for inhumane behavior, is there a way for me to know whether an action I am about to perform is going to be inhumane?