Once Massacre Girl's enters-the-battlefield ability has triggered, it doesn't matter whether Massacre Girl remains on the battlefield. The delayed triggered ability will be created as the enters-the-battlefield ability resolves.
Massacre Girl
If more than one creature dies at once, Massacre Girl's delayed triggered ability triggers that many times.
Creatures that die while Massacre Girl's triggered ability is still on the stack won't cause its delayed triggered ability to trigger since that delayed triggered ability hasn't been created yet.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Chris Rallis
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
