I can not imagine someone thinking a cheating boyfriends infidelity could only be made "pure again" by killing the other girl in his life. Unbelievable. When I was 17 my boyfriend not only cheated on me, but he was a year older and was driving my high school's bus when he met the girl he cheated on me with. He finally broke it off with me, to be with her, on Valentines day! Of all things. And after giving me a huge Snoopy "I love you" card and an expensive bottle of perfume.
He was my first serious relationship and it never occurred to me to harm either one.
I just don't understand where people get these ideas from.
Kill the girl your boyfriend cheated on you with!? Break up with him, since he's the one that betrayed your trust, and find a real man to live with. Don't commit first degree murder and spend the rest of your life caged with girlfriends.
In my opinion, they both are right where they belong. It doesn't matter who each tries to claim was the planner of the murder.
That neither one put a stop to it before it set off, makes them both equally depraved and society the better for them being locked away.