Ruling on working in a casino

Gambling is a major sin which Allah has forbidden in His
Book, and He mentions the prohibition on it alongside the prohibition of
alcohol. Allah says (interpretation of the meaning): 

“O you who believe! Intoxicants (all kinds of alcoholic
drinks), and gambling, and Al‑Ansaab (stone altars for sacrifices to idols
etc), and Al‑Azlaam (arrows for seeking luck or decision) are an abomination
of Shaytaan’s (Satan’s) handiwork. So avoid (strictly all) that
(abomination) in order that you may be successful.

91. Shaytaan (Satan) wants only to excite enmity and
hatred between you with intoxicants (alcoholic drinks) and gambling, and
hinder you from the remembrance of Allaah and from As‑Salaah (the prayer).
So, will you not then abstain?”

[al-Maa’idah 5:90-91]

When Allah forbids a thing, He also forbids helping with it
in any way. 

So it is not permissible to work in a casino because of what
it involves of helping others in sin. Allah says (interpretation of the
meaning): 

“Help you one another in Al‑Birr and At‑Taqwa (virtue,
righteousness and piety); but do not help one another in sin and
transgression. And fear Allaah. Verily, Allaah is Severe in punishment”

[al-Maa’idah 5:2]. 

Whatever money a person earns by working in a casino is
haraam wealth, and it is not permissible for him to make use of it or to
spend it on himself. What he has to do is get rid of this money by spending
it on various charitable causes. 

You have to look for permissible means of earning halaal
provision. If you leave this evil for the sake of Allah, Allah will
compensate you with good. Allah says (interpretation of the meaning): 

“And whosoever fears Allaah and keeps his duty to Him, He
will make a way for him to get out (from every difficulty).

3. And He will provide him from (sources) he never could
imagine”

[al-Talaaq 65:2-3]

And Allah knows best.