Your question seems to seek an opinion. Based upon The Road by Cormac McCarthy, it might seem that to be surrounded by so many instances of horror and the possibilities of evil, that one might believe that thinking about evil simply makes it grow larger, perhaps till it seems to overwhelm the goodness in the world.
In my opinion, I believe that being exposed to anything too much can have negative effects. We can become obsessive about some things, but there are specifics that lend themselves to this kind of thinking. I believe in the idea that when moderation is practiced, there is less of a chance to develop a dependence or obsession about a particular subject or element (drinking, smoking, etc.), unless someone has a physiological predisposition to it.
Thinking of evil does not have the power, I don't believe, of creating more evil. Acts create more evil. In The Road, the unnamed father and son have many experiences with evil people. One might believe that this would be enough to turn someone "to the dark side," but the father does not, and neither does the son. There is choice in all of this. They have to live harsh lives and cannot afford to sacrifice themselves for other, but they do not practice evil on others: they do their best to avoid it.
This experience might make someone believe that thinking of evil can change a person. However, without watching out for evil, the father would never have been able to save himself and his son in this situation:
When he looked up the road rat was holding the knife in his hand. He'd only taken two steps but he was almost between him and the child.
What do you think you're going to do with that?
He didn't answer. He was a big man but he was very quick. He dove and grabbed the boy and rolled and came up holding him against his chest with the knife at his throat. The man had already dropped to the ground and he swung with him and leveled the pistol and fired from a two-handed position balanced on both knees at a distance of six feet. The man fell back instantly and lay with blood bubbling from the hole in his forehead. The boy was lying in his lap with no expression on his face at all….he picked up the boy and turned him around and lifted him over his head and…set off up the old roadway at a dead run...
The father's distrust of the "road rat" is what keeps the two of them alive for one more day. This is how the father and son survive. When the father becomes sick, he has set an example for the boy to be cautious. However, the boy also has learned either to read others and/or to have faith. For after his father is gone from the sickness, as the boy packs up, a man comes along who is one of the "nice" ones. He helps the boy, and welcomes him to his home with his own family.
This man, too, is surrounded by the same terrors as the father and son were, but he has held on to his goodness, even while watching every day for the potential of evil that lies in wait in this apocalyptic world.