What is the significance of screen flashes?

Solution 1:

As stated prior, light blue flashes equate to gaining "good" morality points, the flash is accompanied by whispers and water dripping sounds. The dark flash is the opposite and is accompanied by an ominous tone.

Here is a general synopsis for gaining good points, however read the walkthrough I've linked at the bottom for an in depth map of where and how to gain these points:

• Avoid killing people in general. Unfortunately you are forced to be "stealthy" throughout the game to get the Redemption ending.
• Listen to any conversation that you think your character might gain information about the Metro from.
• Strum every instrument.
• Listen to anyone playing an instrument themselves.
• Don't kill anyone who is surrendering.
• If you think it would be a "nice thing to do", such as following and covering Anna until she is in her perch or shooting the "dogs" off of the "mama bear" at the end of that encounter, then do it. It's very open to interpretation.

In short, listen to every passive conversation going on, do good stuff, sneak around, don't kill anyone.

You can find a decent step-by-step walkthrough on how to get the "redemption" ending in the Steam guides found HERE. The guide gives you ample amounts of specific chances throughout each level to gain moral points and warns of a few chances to lose morale points as well.

My opinion: I hate it when games force you into a certain playstyle in order for you to see a desired ending. This was a problem with Dishonored as well. If I wanted the good ending I could ONLY skulk around and never get into any combat encounters. With Metro it is largely the same. The more you kill, the more likely you are going to get the "bad" ending. I personally simply watched the "other" ending on Youtube and left it at that. I honestly enjoyed the "bad" ending more at any rate.

Solution 2:

It is a bit different from previous game. Alternative ending is really hard to get, because you need to explore all the game and get every special achivements (like going throught the Bridge without killing mutants) to get it.

So now, every action is counting for this ending. Like playing the guitar, finding secret places and so on.

But still there are few inhumane, as you called it, actions. I can't find the link to description of all of them, but indeed there are few such actions. It is about 1 inhumane action for 10 humane ones.

Also, there are always been such devision. Screen flashes blue when you've done good thing, and get darker when you've done something bad. And in both games this affects only ending and nothing else.