Justice versus mercy
Every major conflict in the play comes back to this. Angelo enforces the law without mercy. Isabella pleads for mercy over strict punishment. The Duke tries to split the difference at the end, but the play never fully endorses either position.
Hypocrisy in power
Angelo is celebrated for his virtue before he has any real authority. The moment he does, he abuses it. The play shows that moral reputation and actual behavior are two different things, especially for people in charge.
Sexual power and coercion
Angelo's proposition to Isabella is a direct abuse of his legal authority. He uses the threat of her brother's death to coerce her. The play treats this seriously, not as a subplot, and it shapes every decision Isabella makes afterward.
Appearance versus reality
The Duke disguises himself. Angelo hides his desires behind legal language. Mariana hides in the dark. Almost every character in the play is performing a version of themselves that is not the full truth.
The limits of law
Claudio's crime is real but minor. The law Angelo enforces is technically valid but widely ignored. The play asks what law is actually for, and whether a law that produces unjust outcomes is still worth enforcing.