He grows up in Muggle London believing that monsters are the stuff of fairy tales and Saturday matinees. Ten pence will earn you a respite from the afternoon heat and a chance to put your arm around a girl in a darkened theater.
"Even a man who is pure in heart, and says his prayers by night
May become a wolf when the wolf-bane blooms
And the Autumn moon is bright."

When he joins the wizarding world he learns that some monsters are real.
Third year DADA, trying hard not to snigger in the back row with Tony Stewart, another Muggle-born boy from Gryffindor: "A werewolf can be distinguished from a true wolf physically by several small distinguishing characteristics, including the pupils of the eyes, snout shape, and tufted tail..."

As he grows closer to his son-in-law he realizes that not all monsters are evil.
"Take care of my girls, Remus. And that grandbaby of mine." He grips the man's hand tightly, forcing the words past the sudden lump in his throat.
"I will protect them with my life, Ted. You have my word."
He knows Remus well enough by now to trust that the younger man will do just that. It gives him the strength to walk away.

And at the end, writhing in pain on the cold, hard ground, he comes to understand that the worst monsters of all are quite human.

1 "The Wolf Man" (1941, Universal Studios);

2 HP lexicon