Your concern for your neighborhood is admirable and understandable. You likely have grounds to step in and "just say no" to the buyers in this case and decline their offer, assuming it's not too late to halt the sale. If the deal hasn't closed, the family really can't force you to sell. They could conceivably take you to court to recoup any money they spent during the buying process and possibly pursue other damages, depending on what type of contractual arrangement you signed. If push does come to shove, only a real estate attorney can take into account all the variables and pose a strategy for you to pursue.
You are also asking if a buyer's agent has an obligation to disclose the fact that a member of the buying family is a serial violent person. Answer: probably not. Most buyer disclosure forms are designed merely to assist the seller in evaluating the actual buyers' fiscal wherewithal to purchase the home, not whether a family member is a thug. For example, a standard disclosure form used in Florida asks the buyer, "Is there any past, existing or threatened legal actions (i.e. divorce, collections, judgments, etc.), against you that would prohibit you from purchasing a home? The keyword here is "you," unfortunately, which means the actual buying party behind the deal, not Mr. SWAT Standoff himself, who likely can't keep a job. There's also another common disclosure question that asks, "Is there anything else you feel should be disclosed to a seller that may adversely affect your ability to purchase a home?" While the answer to that, in your view, might clearly be the destructive actions and ensuing legal bills generated by said violent relative, that's really a relative call, especially for an agent and client who no doubt desperately want this deal to fly.
But let's talk about your agent -- assuming you used one -- who is really the person you should expect to assist with such buyer screenings. Hypothetically, a competent seller's agent should be willing to help screen buyers for past criminal behavior or at least show the client how to do so. (Did you do any research on this family yourself?) In fact, several states, including California, require agents to direct homebuyers to resources such as sex-offender databases before the signing of the sale contract or certainly, the closing papers. In your case, if there was no direct connection between this person's criminal record and the actual buyer, your agent may have simply missed the connection, though I must point out that the address of the offender should have matched the buyer's.
Let this be a lesson for all those sellers who are concerned about their soon-to-be former neighbors to screen their buyers. Here's hoping your deal is not irreversible. Good luck!
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