Selling kitchen knives, and giving tips and instruction on where to stab to inflict maximum damage: I'd be very surprised if you're not legally liable for the actions of the third party. But, IANAL.
EDIT: as for free speech, the "imminent lawless action" [1] test is AFAIK the current standard in the US. I'm sure there's case law around this that I'm not familiar with, but I'd argue giving killing tips is likely to incite "imminent lawless action" in the knife analogy.
Selling kitchen knives, and giving tips and instruction on where to stab to inflict maximum damage: I'd be very surprised if you're not legally liable for the actions of the third party. But, IANAL.
EDIT: as for free speech, the "imminent lawless action" [1] test is AFAIK the current standard in the US. I'm sure there's case law around this that I'm not familiar with, but I'd argue giving killing tips is likely to incite "imminent lawless action" in the knife analogy.
1. http://en.wikipedia.org/wiki/Imminent_lawless_action