I guess I just disagree with this concept then. If he sells someone drugs and they OD, he should be prosecuted for selling drugs, not murder. If he sells someone a gun and they kill someone - not his problem and he should enjoy his Christmas party (other than the stealing evidence part, which I'm not really sure how that fits in this analogy). I believe in personal responsibility. Selling guns isn't wrong, killing people is.
The part highlighted is the criminal part. Same as allowing prohibited persons to buy guns (they are prohibited for a reason).
Knowingly and willfully selling guns to Bad People
(tm) is where you get to take responsibility for what Bad Things
(tm) happen with those guns afterward. The BATFEIO is supposed to keep guns out of Bad People's
(tm) hands, not give them guns with pretty pink bows on them.
You can "disagree" with the concept of
Accessory all you want. But you might want to read up on it before you run afoul of it....