a person becomes jealous because they perceive someone else as having or being something they don't believe they think they can have or be, so they feel that they are inadequate, that is why they become angry with that individual and try either
conspicuously or
inconspicuously to
sabotage them because they are trying to take something away from them, to feel equal to them.
A lot of unattractive women feel jealousy towards attractive women because they feel that they themselves can never be attractive, so they
feel inadequate and consequently, they try to
sabotage the attractive girl by excluding her, spreading rumors about her, trying to get her fired, trying to beat her up, etc... but if they got
in shape and changed their whole self-image, the attractive chick would no longer be a threat.