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.

Crimes committed by Grindelwald (or his followers, on his orders) in The Crimes of Grindelwald:

1. Breaking out of MACUSA/international custody


2. Killing a bunch of MACUSA/international lawkeeping wizards in the process of the above


3. Stealing the coach that they were driving with


4. Illegally entering Paris as a fugitive


5. Killing a French muggle family (his followers, on his orders)


6. Breaking/entering and illegally taking up residence in the family's home


7. Killing Credence (/Corvus)'s nurse (his follower, on his orders)


8. Stealing records from the French MoM archives (his followers, on his orders)


9. Covering Paris in magical black cloths (probably violates some vandalism/public safety regulations)


10. Breaking into a tomb and holding a big meeting there, presumably without getting any permits


11. Attacking and killing a bunch of Aurors


12. Unleashing Fiendfyre on Paris

Grindelwald’s crimes:

Grindelwald committed several crimes that are probably illegal, and aren’t just things that might be considered “wrong” but aren’t truly considered crimes. Here is a list of things he’s done that other wizards have been convicted of, so are probably considered crimes by wizarding law in at least one country, and are clearly shown as being done by Grindelwald himself or directly ordered by him.

Murder of other wizards:

Murdering other wizards is certainly considered a crime by the British Ministry of Magic, since Antonin Dolohov was convicted and sentenced for it, and it almost certainly would be also a crime in other jurisdictions like America and France.

“Antonin Dolohov, read the legend beneath a wizard with a long, pale, twisted face who was sneering up at Harry, convicted of the brutal murders of Gideon and Fabian Prewett.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 25 (The Beetle at Bay)}}

He kills several Aurors in his escape from MACUSA custody.

Grindelwald kills several Aurors while escaping. He knocks four of them out of his transport carriage.

GRINDELWALD points his wand at the carriage reins, turning the black ropes into living snakes that ensnare AUROR 1 so he falls from the carriage, back through the night sky, past the broomstick riders.

GRINDELWALD casts another spell so the black ropes of the reins bind AUROR 2 like a chrysalis, launching him forward in the air, then slingshotting him back to knock AURORS 3 & 4 from the rear of the Thestral-drawn carriage. They fall away into darkness.
_{^{- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)}}

Then he strikes down the Aurors pursuing him on broomsticks with lightning.

Still driving the carriage, GRINDELWALD swirls his wand in the air toward the surrounding storm clouds. One by one, forks of lightning strike the broomstick riders, knocking each in turn from the sky.
_{^{- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)}}

He also washes out two Aurors from in the carriage with water, tipping the carriage, throwing them and the water out.

GRINDELWALD appears at the door and nods to ABERNATHY. He throws the door open so the water pours out—along with the two remaining AURORS.
_{^{- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)}}

He succeeds in killing more Aurors with the magical fire.

He kills Aurors with the fire he used at the cemetery.

GRINDELWALD conducts the flames as though leading an orchestra, the Elder Wand his baton, as the forks of fire strike at AURORS attempting to Disapparate or flee.

He ordered Grimmson to kill Irma Dugard.

Grindelwald ordered Grimmson, who was working for him, to kill Irma Dugard.

Fresh from IRMA’S murder, GRIMMSON stands in a covered alleyway beneath a bridge over the Seine. GRINDELWALD appears.

GRIMMSON

She’s dead.

GRINDELWALD walks toward him and halts when they are face-to-face.

GRINDELWALD

How did the boy take it?

Attacking Ministry officials:

Attacking Ministry officials is a crime in Britain, and likely elsewhere as well.

“Marvolo, who had injured several Ministry employees in addition to Ogden, received six months.”
_{^{- Harry Potter and the Half-Blood Prince, Chapter 10 (The House of Gaunt)}}

He attempts to use fire against the Aurors.

Grindelwald attacking the Aurors with fire would be a crime in itself, whether or not he succeeds.

GRINDELWALD sends a wall of flames into the air, pursuing fleeing AURORS.

GRINDELWALD

Play by the rules! No cheating, children.
_{^{- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)}}

Killing Muggles:

Killing Muggles is a crime in Britain, and likely would be in other countries.

“So the Ministry called upon Morfin. They did not need to question him, to use Veritaserum or Legilimency. He admitted to the murder on the spot, giving details only the murderer could know. He was proud, he said, to have killed the Muggles, had been awaiting his chance all these years. He handed over his wand, which was proved at once to have been used to kill the Riddles. And he permitted himself to be led off to Azkaban without a fight.”
_{^{- Harry Potter and the Half-Blood Prince, Chapter 17 (A Sluggish Memory)}}

He ordered his acolytes to kill the Muggles in the house he wanted.

Grindelwald orders his acolytes to kill the Muggles living in the house he wanted to use for his hideout in Paris.

GRINDELWALD and ACOLYTES stand in the street. GRINDELWALD points his cane at a particularly fine house. A clatter announces the arrival of a horse-drawn hearse.

NAGEL, KRALL, CARROW, ABERNATHY, KRAFFT, ROSIER (female), and MACDUFF approach the front door. KRALL opens it with his wand. The ACOLYTES enter.
_{^{- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)}}

He clearly expected the Muggles to be killed, he waits outside until he sees the coffins come out.

GRINDELWALD looks around the street, calm, waiting, tapping on the pavement with his cane.

We see a green flash—the Killing Curse. The door reopens. Two black coffins exit. GRINDELWALD watches as NAGEL and KRAFFT load the coffins onto the carriage.
_{^{- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)}}

Trespass and robbery:

Trespassing into the British Ministry was considered a crime, it’s likely it’s also a crime to trespass into the French one and rob it, especially considering the Matagots guarding it.

“Sturgis Podmore, 38, of number two, Laburnum Gardens, Clapham, has appeared in front of the Wizengamot charged with trespass and attempted robbery at the Ministry of Magic on 31st August. Podmore was arrested by Ministry of Magic watchwizard Eric Munch, who found him attempting to force his way through a top-security door at one o’clock in the morning. Podmore, who refused to speak in his own defence, was convicted on both charges and sentenced to six months in Azkaban.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 14 (Percy and Padfoot)}}

He had his acolytes steal the Lestrange family tree from the French Ministry.

Though it’s never explicitly stated, it’s almost certain that Grindelwald’s acolytes broke into the French Ministry to steal the records on his orders, since stealing the Lestrange family tree fits into his plan to lure Credence to him.

As the RECEPTIONIST gives a Gallic shrug, a genteel ELDERLY LADY crosses into the shot behind QUEENIE. She has a distinctive bag in her hands—we follow her into the elevator—where ROSIER stands waiting. As the doors close, the ELDERLY LADY transforms into ABERNATHY and he pulls out an elaborate box . . .
_{^{- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)}}

They later plant it in the mausoleum in furtherance of Grindelwald’s plan to lure Credence.

ABERNATHY and MACDUFF enter carrying the bag retrieved from the French Ministry and remove the elaborate box, which they plant in the mausoleum to be found.
_{^{- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)}}