The second personality did it to protect main personality. Yes, 2nd personality enjoys killing, but so does Silkie.
Again, I'm not saying that Silkie is making the right choice. (She isn't.) Rather, she's making the choice that she wants to make. It's not surprising that she's sympathetic to someone who doesn't want to kill, but is forced by circumstances to let loose their inner murder-demon.
Remember Silkie's previous self wanted to die, to be erased. She only got reincarnated in this way because the god wants to use her as a murder hobo.
PS: This manga has damn few characters who aren't murderous. It's Shades of Black here.