I gotta go with Shallan on this one. In the Shallan Reading she's in the middle of some bloody corpses ( including her mother ) while her father tells her to forget everything and hugs her, but she says monsters shoud'nt be held in love or something along those lines. There a lots of possibilities: 1) Her father killed those people + mother and by monster she's refering to him 2) Her father killed those people, because the killed his wife in during a break-in or smt
3) Shallan can somehow go berserk when someone she loves is in danger and killed those people because they hurt her mother ( if you keep this in mind, that explains how she killed her father who probably had a shardblade when he hurt her brothers ). Because she killed them she thinks of herself as a monster.
Quote 1:
"Father stepped over the body of a woman in white. Little blood there. It was the other men doing most of the bleeding.
Mother lay face down so Shallan couldn't see the eyes, the horrible eyes."
Quote 2:
"Pretend it never happened," her father whispered, he wiped something wet from her cheek, his thumb came back red. "I'll protect you."
Quote 3:
" No... no... This affection wasn't right. A monster should not be held in love, a monster who killed, who murdered. "
PS: Here's the link : http://tinyurl.com/kwlloo6