I can think of two possible reasons but neither are air tight. However, both are backed up by the fact that he tosses the wand down meaning he did have an intention of giving Spielman the chance to live. Of course the screenplay gives no insight into this but merely states that he tosses the wand after him.
He blasts SPIELMAN magically through the open door, then tosses a wand after him.
Fantastic Beasts: The Crimes of Grindelwald - The Original Screenplay, Scene 15
As SPIELMAN falls, he manages to seize the wand and conjures an invisible Slowing Charm. Sinking slowly toward the sea, SPIELMAN watches his carriage streaking away in the direction of Europe.
Fantastic Beasts: The Crimes of Grindelwald - The Original Screenplay, Scene 16
This is just a play on the common tropes of Sole Survivor and Spare a Messenger so Grindelwald can get the message out that he has escaped. This obviously has its downsides that people will come after him quicker but will also mean he can get the message out quicker to his followers. Also the fact of how he escaped, besting a large group or Aurors in a daring escape is impressive and it would be good to have a third party witness to this saying the same.
Spreading his own message
Towards the end of the film we see Grindelwald wanting to spread his own message that it is the Aurors who are violent not them. Of course he deliberately charges the situation to have the outcome he wants but his message is conveyed nonetheless. This could be why he spared Spielman as the Aurors were attacking him so he "defended" himself whereas Spielman did not attack so he let him live.
But one of the jumpiest young AURORS has made eye contact with the YOUNG RED-HAIRED WITCH. She is angry, as twitchy as he is, fingering her wand.
GRINDELWALD They have killed many of my followers, it is true. They caught and tortured me in New York. They had struck down their fellow witches and wizards for the simple crime of seeking truth, for wanting freedom...
He is deliberately playing on the unstable YOUNG RED-HAIRED WITCH’S feelings. The YOUNG AUROR raises his wand a few inches. He can sense her desire for violence—
GRINDELWALD Your anger—your desire for revenge—is natural.
And it happens. She raises her wand, but the YOUNG AUROR curses first. To the horror of her companions, she falls, dead.
Screams fill the auditorium. GRINDELWALD ascends into the crowd, which parts for him. He kneels and pulls the YOUNG RED-HAIRED WITCH’S limp body into his arms.
GRINDELWALD (to her friends) Take this young warrior back to her family.
The Niffler, unnoticed, wriggles out from beneath GRINDELWALD’S boot and disappears into the crowd.
GRINDELWALD Disapparate. Leave. Go forth from this place and spread the word: It is not we who are violent.
Fantastic Beasts: The Crimes of Grindelwald - The Original Screenplay, Scene 114