償う means to atone for a wrongdoing or make amends for a debt, mistake, or sin. In the lyric 罪を償う様に, it creates a dramatic image of someone trying to pay for their guilt, giving the song a powerful sense of betrayal and consequences.
"ナイフ" (Knife) is a breakup song where the narrator has decided she is not going to cry quietly about it. It opens on a late night in the rain, with her using the rain to hide her tears, and then turns straight to the man who lied to her: "嘘をつくあなたは何を守るつもり" (You who lie, what are you trying to protect?). He thought she would never notice, and she is furious that he took her for a child: "子供だからって馬鹿にしないで" (Don't make a fool of me just because I'm young).
The hook is where the song gets its edge. She gathers up "殺意にも似た愛情" (a love that looks a lot like murderous intent) and promises "裏切りのお返しは高くつくよ" (paying back betrayal is going to cost you). The knife of the title never quite gets used: she says she needs "ナイフ納める場所" (a place to put the knife away), and that she could finish him off any time ("とどめはいつでも刺せるわ"). It is melodramatic in the best visual-kei way. And then the last lines flip it all: before ending things on a rainy night just like the one they met on, she asks for one more night to have him all to herself, "独り占めさせて" (let me have you to myself). Useful words to grab here: 裏切り (betrayal), 約束 (promise), and 許せない (I can't forgive).