It depends on the side it was on, but I wouldn't turn away only for that reason.
If I was shopping for Lugers, and had a choice between a never-unholstered bring-back or one fired twice into a Brit and captured after the war, I would just personally buy the one that had never been out of the holster.
On the other side, I have a very early, issued M44 rifle that I feel just fine about.
I also have a '42 Carcano--an Italian WW2 rifle. No importer's mark, so I'm pretty sure it's a bringback. I think that one might pull even.
I, personally, wouldn't feel good about owning a gun that was used to kill someone I know. Not so much distrusting the gun, but it would be the same as owning the motorcycle they fell off of. Something to fix, respect, and remember, but I wouldn't want that particular object as a reminder.