I’m hoping for a single descriptive word for “people who have been taken advantage of”, but a shorter phrase would be ok.
These are people who now have a lack of trust and are in need of loyalty in their relationships.
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One of these might work:
dupe1 [doop, dyoop]
1. a person who is easily deceived or fooled; gull.
2. a person who unquestioningly or unwittingly serves a cause or another person: a dupe of the opponents.
verb (used with object), duped, dup·ing.
3. to make a dupe of; deceive; delude; trick.
2. a person who is deceived or cheated, as by his or her own emotions or ignorance, by the dishonesty of others, or by some impersonal agency: a victim of misplaced confidence; the victim of a swindler; a victim of an optical illusion.
A word usually carries certain implications with it. Your question seems to be searching for two distinct meanings.
A word or phrase to identify "people who have been taken advantage of…" might be "dupe", "patsy", "stooge", "sucker", "fool", "fallen angel", "mark", "victim", etc.
Someone who, by being duped in the past, is distrustful of others' intents might be a "cynic", a "hard-boiled egg", "doubting Thomas", "skeptic", "churl", "pessimist", "curmudgeon" etc.
And perhaps another for someone who needs loyalty in their relationships.
The word to be used depends on which meaning you want your audience to infer the most.
What comes to mind are such phrases as once bitten/burned, twice shy and a burnt child dreads the fire.
once bitten/burned, twice shy and a burnt child dreads the fire: something that you say which means that when you had an unpleasant experience you are much more careful to avoid such similar experiences in the future.
Alternately, consider victimized, used, put-upon, and suckered.
victimized; used; put-upon; (slang) suckered: (of persons) taken advantage of.
In the way of nouns, consider patsy and fall guy.
The accepted answer is good. But IMO "victimized" is as good, or perhaps an even better choice, because you added:
people who now have a lack of trust and are in need of loyalty in their relationships.
"Exploited" means some has been taken advantage of - "used" - but they are not necessarily aware of the fact, or see anything particularly wrong with it. On the contrary, "exploited" is usually used when the victims don't know they've been used and don't realize any wrong has been done. "Victimized" however, means exactly that - someone who has been made to suffer - been made into a victim.
Note that the choice of word will depend on how you want the reader to view the victims. Words like "dupe", "patsy" and "sucker" imply that the person was foolish. I hear 'sucker' used as an insult or taunt. Such words might be used by the perpetrators of a fraud to devalues their victims (the "marks") in their own eyes.
A word like victim or exploited is more neutral. They are not in some way to blame for their misfortune. The blame rests with the perpetrators.