This is the 3006th most frequent Spanish word.
criminal
A person who commits a crime.
In this sentence, "criminal" is used as a noun to directly refer to a person who committed a crime.
El criminal fue arrestado por la policía.
The criminal was arrested by the police.
Here, "criminal" operates as an adjective describing the nature of actions as unlawful or related to crime.
Sus acciones fueron de naturaleza criminal.
His actions were of a criminal nature.
In this example, "criminal" is again used as an adjective to qualify behavior as unacceptable and illegal.