Currently, the ratio of legal to illegal activity on the bitcoin network has completely changed trend as the former surpasses the former. This was stated by DEA Agent Lilita Infante at the U.S Drug Enforcement Administration.
In my honest opinion I think the rate at which criminals use the bitcoin for their fraudulent activity has reduced simply because of the transparency of the bitcoin as every transaction happen to be recorded on the blockchain technology
Though that doesn't mean the currency isn't used for fraudulent activities as its basically decentralized and anonymous..
The system is still dominated in my own opinion by both criminals and speculators(who keep waiting for the price to rise,rise and rise)