Malvada translates to "wicked" or "evil". It's a strong, dramatic adjective that isn't used in everyday conversation as often as milder words for someone who behaves badly.
In the song it is hurled as a direct accusation: "Vete ya mujer malvada" ("Go away now, wicked woman"), giving the farewell a sharp, almost theatrical edge of betrayal.
The order of events is the whole point. By the time she comes back, he has already finished with it: "Yo no tengo la culpa que me quieras" [It’s not my fault that you love me], he tells her, and then the reason, "Cuando yo ya me he olvidado de tu amor" [When I’ve already forgotten your love]. That is not a plea. It is a timetable.
Why she lost him gets exactly one couplet: "Siempre quise quererte con locura / Pero tú me has pagado con traición" [I always wanted to love you madly / But you’ve repaid me with betrayal]. The chorus is far less polite, and it comes back until you can sing it yourself: "Vete ya mujer malvada / A engañar a otro hombre con tu amor" [Go away now, wicked woman / To go deceive another man with your love]. The injury is a kiss, which is a good touch: "Con tus besos has roto mi corazón" [With your kisses you’ve broken my heart]. When she turns up expecting to be waited for, the reply is flat: "Te equivocas mujer, porque es ahora / Que yo, ya no te amaré" [You’re wrong, woman, because it’s now / That I will no longer love you]. All of it rides an accordion figure you cannot sit still to, and halfway through someone shouts "¡Con sentimiento!" [With feeling!] over the break, which tells you how seriously Los Ángeles Azules expect you to take your own broken heart.