Not sure how I'd deal with a scenario such as described in the OP. It'd be a bummer on my conscience (regardless of media).
Why not make the question even closer to home? What if it were a friend or family member that abused the "tool" you made? Maybe on him/herself or someone else? That'd be a tough bit to live with, no? Even more pointedly, would there be nuances to the wretchedness if the dire deed was done with re-purposed kitchen cutlery verses a knife truly designed to be a weapon, such as a dagger or some such? For my part, I believe there would be.
I cannot equate someone re-purposing a baseball bat for ill will with that of using a combat weapon, such as a machine gun, for mass murder.
Can we always obviate ourselves from responsibility?
What if we suspect or have reason to believe or know there was imbalance (toward self or others) in a friend or family member...or client? Do we still go through with the sale, the gift, the housing of weaponry in their proximity? For instance, I know a guy whose daughter was suicidal (she was an adult survivor of child molestation) so he got rid of all his hunting guns. His wife cried (for his lifestyle sacrifice...and their daughter's dilemma) as he carted his guns away. This was a guy who kept his firearms carefully locked in one cabinet and his ammo in another. He simply was worried it would not be enough and did not want to take the risk.
When I was a young lad in the seventies making jewelry, of all things, I had a guy who wanted me to make brass knuckles (out of sterling) with skulls on them because it would be cool to leave them as imprints on people's faces when he got in fights. I did not take the commission. On another note, I knew a guy that lost a finger wearing one of my rings when the stone got caught on a stable wall as a horse fell against it. That was a bummer, certainly not my fault, but a bummer. You can bet ever since then I told customers to be careful while wearing jewelry. (I don't wear any.)
My hat's off to all you good bladesmiths and knifemakers, as well all folks who are wrestling with the anguish of violent tragedy.