Exiting a contract - discovered other party sells drugs

Hello, I am currently developing a web site for a couple. A month or so into the contract, I learned that their side business is dealing drugs out of the home that their business is also based in. I am extremely uncomfortable with this. Is this a basis to exit the contract? Any funds I have received to date ($10,000) I have spent on the site except for $1,500 which I am willing to return. I cannot return the funds I have already spent on their site. Is it legal to cancel the contract and return the unspent funds? I am not looking to report them or cause any problems, I just don't want to be involved with people who break the law. Thank you!