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.

When subpoenaed, you'd be obligated to tell the court who you gave the knife to.

But if you'd like to tell the fall, I'm sure some prosecutors wouldn't dig too hard to find the guilty party.