If you know who and where David Martinez really is, you can have him arrested for his threats to you and your family or you can have him served and sue him in small claims court for the return of the money he owes you.
Unfortunately, however, if this is just some man you met online but have never met in person, he may be using phony contact information. If that's the case, finding him and getting him brought to justice so that you can avail yourself of legal remedies would be a job for law enforcement.
If law enforcement cannot turn him up, you're not completely out of luck. When you're the victim of a scam, your unremibursed losses are tax deductible. So even if you and the police are unable to find this guy to sue him or have him prosecuted, you can get some money back in the form of a tax deduction when you file your 2016 taxes.