If you must put the blame, full or part, on the firearm, consider it an act of rehabilitation. Where once it committed such great evil, with Christian love and influence it now contributes to a peaceful society.
Tools are just tools, when we attach the action of the owner on them we lessen the stigma on the criminal. The SS officer my grandfather took his Luger and knife from could very well have been assigned to a camp and those weapons used to commit atrocities during the Nazi rule. Since 1945 neither have been involved in a murder. My Grandfather, may he rest in peace, was a man of good will and good intentions. The SS officer was a man of doubtful morality. They both used the same tool for very different purposes. To paraphrase Shakespeare- the evil is not in our tools but in ourselves. Attaching significance to the past use of the tool implies the current owner is somehow responsible for the acts of the previous owner. A bit of mental gymnastics I fail to understand or accept.