Crime can be funded with anything: from cash,gold,silver to bitcoin. So all these methods can be hardly traced,not only bitcoin. I don't see any major reason why bitcoin price would decrease if it is used in crimes. Most surely us dollars are used in crimes daily and the price doesn't fall right? 
You are right, There is no sense when we said that bitcoin price would decrease because if it is used in crimes. Because peoples can use dollars or any real or digital currency for this things. Maybe bitcoin is most used for crimes, because it's more secure and anonymous. It used in darknet too. So killers would sure want to use it than other any currencies.
I think legal trades don't decrease the amount of illegal trades. Surely the number of legal trades increased a lot, but illegal trades increased as well. Bitcoin has the price it has today not only by the good people, but because the dark market over it too, we need to be aware about it.
Bitcoin is used by the good and by the evil like fiat and like any other tradeable item. We just need to make sure we are using it for the good, otherwise there could be problems with authorities in real life.