Why didn't Graves finish Newt?

Question

In the fight in the subway, Graves attacks Newt (when lying on the floor) with spark attacks.

Graves is definitely a more powerful wizard since he's actually

The dark wizard Grindelwald in disguise.

Why didn't he finish Newt when he had the chance?

Answer 1

The script suggests that Graves is taking the opportunity to work out some issues (bold is my emphasis, italic from the text):

Finally, almost bored by Newt's efforts, Graves casts a spell that ripples along the train tracks and down the tunnel, finally blasting into Newt, throwing him high into the air.

Newt lands on his back and Graves immediately sets upon him, casting spells in a whip-like motion with increasing vigor. Graves's immense power is evident, as Newt writhes on the ground, unable to stop him.

[...]

Graves continues to whip Newt; a manic, crazed look in his eyes.

^{Fantastic Beasts and Where to Find Them: The Original Screenplay}

Since Newt has been a persistent thorn in his side for some time now, it seems as though Graves is taking the opportunity to get his brutal revenge, a classic Bad Guy mistake.

Answer 2

To people like Voldemort and Graves, killing is just not about casting a curse and ending it. Usually they give out pain, some soliloquies, some demeaning dialogues about the people they're fighting against and lots of self praise. It's just how they are.

Using a killing curse makes you a murderer, but torturing and demeaning your opponent and then killing them makes you a Voldemort or Graves (maniacal power hungry villains). Which is what defines their character.

He probably intended to kill Newt once hes done thrashing him about, but never got around to it due to his elaborate revenge sequence and the good interruption.

Answer 3

My impression was that, initially, he was trying to get the wand away from Newt, and got frustrated that Newt was able to cling to it (he was desperately holding onto his wand with both hands), and got somewhat fixated and frustrated by that when he could have easily have just finished him off, had he not gotten fixated on not being thwarted.

Answer 4

He likely didn’t think Newt was any threat.

It’s likely that Grindelwald hadn’t considered Newt any sort of threat to him, and didn’t think Newt ‘worth’ killing off. Grindelwald-as-Graves was easily able to fight off several Aurors all at once.

“Graves strides confidently back along the platform, firing spells at both groups of Aurors facing him. Spells fly back at him from all angles, but Graves parries them all. Several Aurors are sent flying—Graves appears to be winning . . .”
_{^{- Fantastic Beasts and Where to Find Them (The Original Screenplay)}}

He’d seen Newt before then and had read Newt’s file - he likely wouldn’t consider a Magizoologist someone who could defeat him in a fight.

“Graves examines the file on his desk.

GRAVES
You were thrown out of Hogwarts for endangering human life—

NEWT
That was an accident!”

Newt noticed that Grindelwald doesn’t understand things he considers simple, which likely included Newt himself.

DUMBLEDORE
(overcome)
How in the name of Merlin did you manage to get . . . ?

The Niffler pokes its head out of NEWT’S jacket, sad to see the pendant go.

NEWT
Grindelwald doesn’t seem to understand the nature of things he considers simple.
_{^{- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)}}

Newt’s Niffler was able to steal the blood vial from Grindelwald by avoiding notice, and it’s likely Grindelwald similarly considered Newt simple and disregarded him somewhat.