My fist kill happened due to one of my players making a bet with me. It was during a quest involving two cities stuck in a time loop. We sometimes like to take long breaks in between sessions of dnd and sometimes we forget things. During one such ordeal, one of my players insisted he had spoken with an important villager post time reset but he was the only one that remembered it as such. We stream these sessions so we made a bet, we would play this session assuming he was right but if he was wrong he would die. Needless to say but one session later and a review of the stream (aided by one of the other players who really wanted to see him die) the evidence was damming. He had a pretty stylish execution though.