For starters, you can't* contract to do something illegal, which assault obviously is.

Also, you can't make an "immoral" contract, even if it's not technically illegal.

Further, it was almost certainly signed under duress. Even if that can't be proven, it won't be hard to show the involvement of duress's weaker cousins, undue influence and facility & circumvention.

Finally, on a much more technical point, it sounds as though there's no consideration (i.e. she's not getting anything in return). In Scots law this isn't a problem as we enforce unilateral promises, but English law doesn't, and US law is generally based on English common law. It may be that some states have abolished the requirement for consideration, but assuming that this state hasn't, and that there was in fact no consideration in this "contract", then this would be a technical flaw which would invalidate the agreement even if it weren't so profoundly disgusting.

*What I mean is that if you do, a court will regard the agreement as a nullity. A contract is an agreement which the courts will enforce, so this is an agreement but not a contract.