Malamente is an adverb that literally means "badly". While you might often hear the word "mal" (bad), malamente is a more intense and stylistic choice that ROSALÍA uses to set the whole tone of the song.
The song's subtitle is "Augurio" (Omen), and the repeated chant of malamente creates a haunting feeling of a bad premonition. It's not just that something is bad; it's that things are unfolding in a bad way, as if destined by fate.
ROSALÍA drops us straight into a scene of dark omens and gut feelings in MALAMENTE (Cap.1: Augurio). Right from the start, she knows disaster is coming: "Ese cristalito roto / Yo sentí como crujía / Antes de caerse al suelo / Ya sabía que se rompía" ("That little piece of broken glass / I felt how it crunched / Before falling to the ground / I already knew it was breaking"). From flickering hallway lights to an uneasy night full of moons and stars, everything around her signals trouble. She even describes a nightmare about crossing a bridge that wobbles the faster she moves ("Cuanto más quiero cruzarlo / Más se mueve y tambalea").
Yet instead of hiding away, she heads right out into the dark. She puts on her jewelry for protection: "En la manita los aros brillando, en mi piel los corales" ("On my little hand the rings shining, on my skin the corals"). She also invokes God with the word undivé (a Caló term from the Romani language spoken in Spain for God), chanting "Me proteja, me salve / Me ilumine, me guarde" ("Protect me, save me / Illuminate me, keep me safe"). By the end, she makes it clear she won't waste another second looking back or dwelling on an ex: "No voy a perder ni un minuto en volver a pensarte" ("I'm not going to lose a single minute thinking about you again").