What is the effect on the antithesis in "So sweet was ne'er so fatal. I must weep, But they are cruel tears. This sorrow’s heavenly, It strikes where it doth love"

Othello is talking to himself while he watches Desdemona sleep. He is about to kill her and is justifying his actions to himself. He starts dealing with opposites to describe his internal dilemma of emotion so "sweet yet fatal" / killing what he loves like God in the Old Testament.