The title of Fantastic Beasts: The Crimes of Grindelwald implies that Grindelwald is going to go on a massive killing spree or embark on some sort of major criminal endeavour.
However, if I recall correctly, he appears to spend most of the film plotting, networking and trying to capture Credence. Hardly the actions of a terrible despot. I didn't see much in Grindelwald's behaviour that would merit this description by Rita Skeeter (years later):
The name of Grindelwald is justly famous: in a list of Most Dangerous Dark Wizards of All Time, he would miss out on the top spot only because You-Know-Who arrived, a generation later, to steal his crown. As Grindelwald never extended his campaign of terror to Britain, however, the details of his rise to power are not widely known here.
(Deathly Hallows, Chapter 18, The Life and Lies of Albus Dumbledore).
The only things which could be considered illegal that I spotted were the murders of the Muggle family in the Paris apartment (which I don't believe Grindelwald committed himself anyway) and the banned meeting at the end.
What did Grindelwald do in this film that could be considered criminal?
Of course, he could commit other horrific acts in future films. However, I'm interested purely in events which take place in The Crimes of Grindelwald. Anything that he did which is against American, British, French or International wizarding law (or could reasonably be considered to be so) counts. Acts by henchmen do too, I suppose, but only if they were done on Grindelwald's orders.