Murderers like to keep souvenirs of the people they killed.
Some murders like to keep teeth or fingers. But some like to keep pictures of them.
They are mementos. It’s a common sociopathic murder trait.
Yes I am calling this soldier a murderer, he has all the traits of a murderer right down to him not willing to part with his trophies. This is why he buried his trophies and didnt destroy them.