The Confessions of Marcus Hutchins, the Hacker Who Saved the Internet
wired.com
wired.com
TL;DR: Kronos was a banking trojan that stole people's credentials/money. Marcus wrote part of it. That's why he was eventually arrested, years later.
However, after he'd written it, he started working in the White Hat arena, and just happened to be the guy that figured out the killswitch for WannaCry, which, at that time, was the largest outbreak of malware, evar.
Seriously though, download/install Brave and reload the article. My TL;DR pales in comparison to the details.