The word is from an ancient Arabic one meaning a hashish-eater, because your medieval Arab hitman, being a sensitive chap, couldn't bring himself to do the deed without getting high on the drug, his victim usually being some nobleman, and the Arabs were respecters of rank. It was not often that assassins were called on to dispatch a mere commoner. We have kept that sense of the word in English - our assassinations have traditionally been of royals, aristocrats or leading politicians. The killer of Gianni Versace was described in some papers as an assassin, and this seemed proper, since fashion designers are now venerated as highly as presidents and kings. I'm not sure that someone who mows down a small child deserves the dignity of the word. But then the child may not have been his intended target, in which case he was not even the murderer that the Sun also described him as, for murder implies intent. The London Standard was committing a tautology when it said the boy "may have been murdered deliberately".
Not that murder is much of a word nowadays. A piece in the Independent last week was advising parents how "to get through the summer holidays without murdering your children". No one, except perhaps Tin-Tin's friend Captain Haddock, ever used the word assassin as a joke.