This seems to affect the teens and even some of early twenties age groups. Why? Is the preoccupation actually real (as opposed to fashionable affectation) and is it dangerous?

I'ld think the preoccupation is just them finding out about the world.
The vast majority decide being "good" leads to a fuller life than being "bad".  
Important they make their own decisions in life as early as is reasonable with the knowledge of the  reality and consequences of evil.
eg, my ex was walking with one of our sons. They passed a druggie who was "out of it".
My ex pointed the druggie out to my son and said "Look, there is what someone on drugs is like."
No need to explain anything to him, an excellent lesson in evil right there.


A preoccupation with good would be preferable and if anything beneficial.
The young people I know are very good people. Sure they play "bad" computer games, but overall they lead good lives.

The media highlights the very bad ones. Thgey are the minority.
Yesterday I was at a basketball stadium where many young people play many games of basketball. That's a good thing.

I haven't noticed this preoccupation.

many people my age dont even believe in the concept of evil...