The Evil One
From Middle English sinne, synne, sunne, zen,
from Old English synn ("sin"),
from Proto-West Germanic *sunnju,
from Proto-Germanic *sunjō ("truth, excuse")
and *sundī, *sundijō ("sin"),
from Proto-Indo-European *h₁s-ónt-ih₂,
from *h₁sónts ("being, true", implying a verdict of "truly guilty" against an accusation or charge),
from *h₁es- ("to be");
compare Old English sōþ ("true"; see sooth).
Doublet of suttee.
Baháʼís consider humans to be naturally good, fundamentally spiritual beings.
Human beings were created because of God's immeasurable love for us.
However, the Baháʼí teachings compare the human heart to a mirror, which, if turned away from the light of the sun (i.e. God), is incapable of receiving God's love.
It is only by turning unto God that spiritual advancement can be made.
In this sense, "sinning" is to follow the inclinations of one's own lower nature, to turn the mirror of one's heart away from God.
One of the main hindrances to spiritual development is the Baháʼí concept of the "insistent self" which is a self-serving inclination within all people.
Baháʼís interpret this to be the true meaning of Satan, often referred to in the Baháʼí Writings as "the Evil One".
Watch over yourselves, for the Evil One is lying in wait, ready to entrap you. Gird yourselves against his wicked devices, and, led by the light of the name of the All-Seeing God, make your escape from the darkness that surroundeth you. — Baháʼu'lláh
This lower nature in humans is symbolized as Satan—the evil ego within us, not an evil personality outside.—ʻAbdu'l-Bahá