Who creates computer virus, trojan, malware and ransomware, and why?

The motivations for releasing malicious code onto the public Internet fall into four broad categories:

1. Political ambitions of a nation/state: Good examples are NotPetya and Shamoon, both of which are designed to destroy data and computers.
2. Financial gain by criminals: A good example is Ryuk, a form of ransomware. Another is Zeus, a banking Trojan. Also, Mirai is a piece of malware that was used to conduct online advertising click-fraud in addition to causing the biggest DDoS attacks in history.
3. Political activist agenda: Otherwise known as vigilantes or those practicing civil disobedience, they often deface websites or launch distributed denial of service (DDoS) attacks. A classic example is the Morris worm, which was designed to call attention to insecurity of networked computers.
4. Self-centered goals, like bragging rights or curiosity: A good example was the Melissa virus, a mass-mailing macro virus from 1999.