You absolutely can be, especially if you knew, or should have known, that the knife was likely to be used illegally.
While a bit more extreme than your example, there have been multiple cases where the parents of a school shooter have been held responsible because they provided access to a weapon when there were warning signs.
On the less extreme end of the spectrum, this is the same reason why you have to pretend that you are buying a "water pipe for tobacco" and not a bong if you don't want to get kicked out of the headshop (in places where that is still illegal).
3 year before the murder: You are probably fine, IANAL
10 minutes before the murder: Expect to get an accusation of accessory to murder, conspiracy to murder and a few additional tomes of the penal code. We all know you are innocent, but you should better find a good lawyer just in case instead of wasting your last free minutes arguing on the internet.
While a bit more extreme than your example, there have been multiple cases where the parents of a school shooter have been held responsible because they provided access to a weapon when there were warning signs.
On the less extreme end of the spectrum, this is the same reason why you have to pretend that you are buying a "water pipe for tobacco" and not a bong if you don't want to get kicked out of the headshop (in places where that is still illegal).