Board Thread:Wiki/@comment-24930713-20160109014750/@comment-26932251-20160110042925

Also, Annie may have taken that specific gear because Marco was the one whose death she was most directly responsible for until she goes to town on the Survey Corps. Maybe it was more visceral for her to murder another human being directly with her own hands when he's crying mere inches away from her than when she was in a position to bat away human beings like mice.

What I'm saying is that, as is seen in serial killers in reality, maybe Marco's ODMG was also a 'souvenir' to her.