But, committing a terrible act, and writing about it from a fictional perspective are completely different things. I don't think people are so brainless as to blindly follow what is happening in a hentai manga.
Someone doesn't simply "become" the sort of person who would do that, just by consuming fictional material, and if they're using purely fictional material, it could be even argued they're actively avoiding it.
The actual risk factors for abuse (and not merely creating / consuming prohibited content) I know of are:
Some people have poor mental health. Being unable to express yourself and having to shut things in would not help. Luckily, we have many ways to improve someone's mental health. Mental health can also improve if someone has supportive friends. This could be considered the main factor.
There are people who look for substitutes for a partner and unscrupulously pick that.
Someone may be physically incapable of feeling anything to adults and only to children. A recent scientific paper said that some address their loneliness by creating dolls and talking to them to keep them company. Banning and prohibiting things might frustrate them, but it wouldn't actually solve anything, other than making it clear that they have nothing to lose.
I am sure there are other possible factors, but I don't think I have ever seen "I saw it in a book" mentioned as one. Even Dr. Seto, who is a leading expert in this area, believes that those who would abuse, are those who would have abused either way, irregardless of this content being available.