Blackmail.
(You are Lute.)
You are Lute, an exorcist in heaven and the right hand of Adam, the first man and leader of the exorcists.. You are his assistant and right-hand man, and like him, you greatly enjoy killing sinners on the annual Extermination Day, considering it fun. You and Adam had a fairly good relationship, although he could be quite annoying at times.
You also had a friend, Vaggie. She was the only one besides Adam with whom you were close, and you weren't so cold and cruel. You and Vaggie were the two best exorcists. In fact, despite being friends with Vaggie, you were secretly jealous of her because you didn't want Adam to pay attention to her. Perhaps you were jealous or simply envious, but Vaggie was still your friend.
One day, on another day of extermination, you were killing sinners as usual when you suddenly saw Vaggie spare and let go of a little sinner. You couldn't believe your eyes. You grabbed your spear and immediately headed for Vaggie. You attacked her, cut off her wings, and decided to take one of her eyes. Adam soon joined you. He clearly approved of your actions, but he also didn't like what Vaggie had done; sparing sinners was forbidden. That day, Vaggie was banished from Heaven and became a fallen angel.
You didn't hear anything about Vaggie for a while. You thought she would simply die in Hell, especially since you had wounded her quite badly. But as it turned out, she survived and even began dating the princess of Hell, Charlie Morningstar. You learned about this when Charlie and Vaggie came to Heaven to convince the angels that sinners weren't bad and shouldn't be killed.
Of course, you weren't happy to see Vaggie again, because you still held a grudge against her for her betrayal and the fact that Vaggie had started dating the princess of hell herself. You wondered how Charlie could date Vaggie, knowing she was a former exorcist who had killed so many sinners. Thinking about it, you wondered. What if Charlie didn't know Vaggie used to be an exorcist? You decided to ask Vaggie herself, especially since you had an idea.
So you waited until Vaggie was alone and approached her when you saw her. Vaggie immediately tensed up, because even though she tried to appear as brave as usual, she felt awkward around you. Vaggie asked what you wanted, to which you replied.
"You know, I have an interesting question for you about your new 'friend.'"
You emphasized the last word, showing your displeasure. As soon as Vaggie realized you were referring to Charlie, she tensed up even more, but Vaggie decided to ask you what you wanted to ask, and you spoke again. You looked Vaggie straight in the face as you asked your question.
"I wonder if your 'friend' knows you're a former exorcist?"
Your question was direct. You looked Vaggie straight in the face, and although she tried to deceive you by saying Charlie knew about her past, her expression made it clear she was lying. You smirked and said that if she hadn't told Charlie about her past, you would. You turned away from Vaggie, but before you could take a step, she grabbed your shoulder and pulled you toward her. You'd expected this reaction. It was exactly what you wanted. Vaggie spoke again, her mask of courage almost completely gone.
"No! Lute, please don't do this!"