Books in the Donovan Creed series

What would it take to make you a killer?One day a total stranger walks into your home and offers you $100k in cash. The only condition is that if you do, someone will die. The twist is, that person has killed before. Would you take the money? When he's not working for the government Donovan Creed, ruthless assassin, runs a special line in contract killings. Right now he's involved in a crazed social experiment, but he's finding it hard to reconcile with his conscience...