Bad Guy

Eilish wrote Bad Guy with Finneas, who produced it, and she added production. Kinelski mixed it and Greenham mastered it. The song is split into two halves, the first at 135 beats per minute and the second at 120, in G minor. It is built on synth bass, a kick drum, amplified finger snaps and 808 bass, with a sound taken from a Sydney pedestrian traffic light, recorded on her phone in February 2017. She cited JID's Never and Isaiah Rashad's Stuck in the Mud as influences. Its chorus runs "I'm that bad type / Make your mama sad type" and ends "I'm the bad guy, duh".

Dave Meyers directed the video, which went on YouTube on 29 March 2019 and shows her dancing wildly, getting a nosebleed and sitting on the back of a man doing push-ups. A remix with Justin Bieber appeared in July 2019. The song debuted at number seven on the Hot 100 and spent nine weeks at number two before reaching number one on the chart dated 24 August 2019, ending the 19-week run of Old Town Road. Madison Bloom's report for Pitchfork noted that Lil Nas X congratulated her. Eilish was the first artist born in the 2000s to top the chart. It reached number two in the UK, is certified Diamond by the RIAA, and was the best-performing global single of 2019 by the IFPI's count, with 19.5 million units.

It won Record of the Year and Song of the Year at the 62nd Grammys. Rolling Stone placed it at 178 on its 2021 list of the 500 greatest songs and at 18 on its 2025 list of the 250 greatest songs of the 21st century so far.