All words have been used before. If I was looking for a word that no one else has ever used, I'd soon be speaking and writing in my own language. I don't use text of others (unless I'm quoting them) in my writing...but sometimes 'murdered' isn't a strong enough word; but with little look in a dictionary or thesaurus I could choose the word that is more fitting to MY uses.
for instance.....Murder (happens a LOT in my stories), but I don't want to always use that one word, it would be boring...so I may look it up to find something that fits my scene at that moment, and I'd find:
http://thesaurus.reference.com/browse/murder
abolish, asphyxiate, assassinate, behead, blot out, bump off, butcher, chill*, cool*, decapitate, defeat, destroy, dispatch, do in, drub, dust off, electrocute, eliminate, eradicate, execute, exterminate, extinguish, finish, garotte, guillotine, hang, hit*, knife, knock off, liquidate, lynch, mangle, mar, massacre, misuse, off*, put away, rub out, ruin, shoot, slaughter, slay, smother, snuff, spoil, strangle, take a life, take for a ride, take out*, thrash*, waste*
* = informal/non-formal usage
All of these can be used to express Murder to different degrees and in different situations.
I distinctly remember teachers and mentors telling me that to NOT use these tools as a writer is to box myself into mediocrity....
I look at some of my 'pre-tool' use writing and it's HORRIBLE. Same adjectives used over and over and in one...the word 'murder' is used 47 times. Since then, I've edited it and have removed more than half the repeated use of 'murder' and, depending on the scene, used alternate words that expressed the type of murder (asphyxiated, riddled with bullets, pin cushion stabbing, to name just a few)