Do you have any reference to support this? Or is it just personal fantasy? If you look at the fbi.gov article mentioned (I linked it in a lower comment as well), you may notice that the hitchhikers were most often the victims: The victims in these cases are primarily women who are living high-risk, transient lifestyles
That you nevertheless think this statement to be true - I think rather supports my point about it being a question of culturally conditioned fear and stereotypes.
Anyway, whether hitchhikers are more often victims has no bearing on the question of whether picking up a hitchhiker is more likely to be dangerous than encountering some random person at a convenience store. Most people are quite certain they aren't going to murder a hitchhiker, so they don't worry about that in deciding whether they'll pick one up.