Where do we end up? Anyone who sells a knife is charged for any murder committed with that brand of knife. Does that mean anyone buying a knife has to sign a form releasing the company from legal issues that may arise?
We could split the difference and just assign liability for the ones done by serial murderers and people who show up asking for knives that are good for murdering. Knowing that your product is going to be used for some purpose and providing it anyway makes you an accomplice.