Sue the farmer who grew the corn that the distiller made into whiskey that the driver drank before running into someone on a park bench. It's the farmer's fault, right?
Well, no. Did the firearm malfunction? Did parts of the firearm fail, structurally? If not, then the product performed as it was intended to perform.
How that performance was applied is dependent on the actions of the user.
There are volumes of State, Federal, and local laws which deal with the improper use of, or even just modifications to, a firearm.
If someone used that product in contravention to those laws, better take it up with them.