Name the firearm, weapon that has not caused "pain and suffering for others" somewhere at some time. They also put an end to the users of some of those "dirty" weapons.
My argument is an odd one, I'll grant that much. My position is based on perhaps a flawed morality. Nonetheless, and as an example, I have zero issues collecting Lee Enfield rifles I am confident that the rifle itself was never used to oppress/destroy an unarmed noncombatant. At the same time I recognize the fire bombings allied forces undertook over Germany killing untold noncombatants. I justify that action in two ways; firstly, it was Germany who initiated firebombings of citizenry as early as WW1 and continued to develop the technology through WW2. Secondly, these noncombatants (German) were the enemy. They were Nazi's who had supported that regime with no indications that the German policy was going to be restricted or reversed on its own, or under its own accord. Therefore, their destruction is somehow justified in my mind.
I cannot justify (to myself, I'm not passing judgement on those who do collect.) owning a rifle that is linked through proper provenance to having been in the hands of a guard at any of these German death camps...similarly, I can't justify owning collecting a Samurai sword. It is a fact, that these weapons are "tested" on human beings who could not defend themselves.(the blades, not the Gewher's)
Would a wallet made in a death camp (Buchenwald) with an interesting tattoo in the leather be worth collecting? I have no doubt. I for one would wish to see it put to rest with its true and original owner. That is a more fitting end to the ever living history rather than having some sorry individuals mortal remains being past about among the collectors and money changers of this world. Just further insult to injury in my mind. Clearly, your mileage varies?