be - 'tra (-e)l \ noun
Betrayal's root is betray, which comes from the Middle English word bitrayen — meaning "mislead, deceive." Betrayal has to do with destroying someone's trust, possibly by lying.
Synonyms: perfidy, treachery, treason, knavery, subversiveness.
'per - fe - de \ noun
The noun perfidy means "deceitfulness" or "treachery," so it's not just being mean, but deliberately betraying a trust. The Latin root of perfidy is made up of per, or "through," and fidem, or "faith." So in order for perfidy to happen, there has to have first been a sense of faith in place, which was then broken or betrayed.