I believe I understand this.
If John Dillinger was touting the effectiveness of the Thompson SMG, I'm pretty sure that I not only would not buy something on the recommendation of a murderer, but would not buy from a company that used a murderer as a spokesman.
So the question here is, was this letter written before or after he murdered the people (in the line of duty, of course)?
If it was before, and they are still using him, that's bad. If it was before, and they dropped him like a hot rock but (because once it's on the net it's there forever) someone dredged the old ad up, it doesn't matter.
If, however, H&S decided to use this murderer as a spokesman after he committed his (legal, in the line of duty) murder, then yes, screw them and their company.