[dwattttt]

And if you grabbed the knife and went on a violent spree, I'd absolutely expect the knife manufacturer to refuse to sell to you anymore.

The knife manufacturer isn't obligated to sell to you in either case, I'd expect them not to cut ties with you in the self defence scenario. But it is their choice.

[zephen]

The knife manufacturer would be more than happy to continue to sell to you, except for that minor little detail that you're in jail.

[gpm]

Any knife vendor who

1. Found out you used their knives to go murdering

2. Sells knives in a fashion where it's possible for them to prevent you from buying their knives (i.e. direct to consumer sales)

Would almost certainly not "be more than happy to continue to sell to you". Even if we ignore the fact that most people are simply against assisting in murders (which by itself is a sufficient justification in most companies), the bad PR (see the "found out" and "direct to consumer" part) would make you a hugely unprofitable customer.