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.