I'm sorry, but the song "Mr. Brightside" is originally performed by The Killers, not Hayley Kiyoko. Hayley Kiyoko is an American singer-songwriter known for her own music, not for covering this particular song. However, I can explain the meaning and story of "Mr. Brightside" by The Killers if you'd like. "Mr. Brightside" is a song about jealousy and insecurity in a romantic relationship. The lyrics depict a man who suspects his partner of cheating on him. He recalls a previous encounter where it all started with a kiss, but now he finds himself feeling sick and trapped in his own thoughts. He imagines her with another man, feeling consumed by jealousy and unable to escape these thoughts. The line "Jealousy, turning saints into the sea" suggests that his jealousy is overwhelming him to the point where his own self-control is slipping away. Despite the pain, he realizes that he cannot escape from these feelings and continues to picture their infidelity. The song conveys the emotional turmoil and struggle caused by insecurity and suspicion within a relationship.
