A person who uses his expertise to gain access to other people's computers to get information illegally or to cause damage, is a:

A hacker is a person who uses technical expertise to gain unauthorised access to others' computers to steal information or cause damage.

The term precisely fits someone breaking into systems illegally for data or harm.

An analyst studies and interprets data or systems for legitimate insight, not for illegal access.

A messenger simply carries or delivers messages and has nothing to do with intrusion.

A spammer floods inboxes with unsolicited bulk email but does not necessarily break into computers.

Ethical or white-hat hackers instead test systems with permission to strengthen their security.