Nine shots do sound painful to me. I doubt they were done within seconds of each other. I think they did it to torture him and/or trying to make him open the safe and other reasons. He may have opened it we do not really know, they could have taken stuff and closed it back. I do think its possible that he could have been threatened with the knife in bad ways that I will not mention, even if it was not used on him.
The knife was put in concrete like the guns so it makes it suspicious as to why they would do that. There was one item in each bucket it sounds like from discovery, so they specifically put the knife in a bucket like they did the guns. It could have just been used on other things and had some fibers or skin cells on it also I guess.
Seems like a lot of photos to take of one item in each bucket though, to me. Maybe I am interpreting it wrong.