Why does Graves want him dead?

Question

In Fantastic Beasts and Where to Find Them MACUSA captures Newt and Goldstein and, accusing them of being responsible for the death of Senator Shaw, Graves orders them killed.

Why? How does he benefit from their deaths? Newt has demonstrated his ability to capture and control an obscuriel, which skill would seem useful to Graves. Graves is trying to pin blame for Shaw's death on them, but as the actual culprit (an out-of-control obscurial) has not yet been caught, he seems to be running a significant risk that the creature kills other people after he has had Newt and Goldstein executed, thus revealing that they were not responsible and he had killed innocent people.

Was he planning to continue to blame them indefinitely for having introduced a murderous creature to the city? Given the nature of the killer, isn't he taking a lot of risk that somebody else connects the dots to its true origin?

Answer 1

Graves outspoke. He said referring to the obscurial Newt kept trapped:

So it’s useless without the host?

Then Newt confronts him by saying

That is a parasitical magical force that killed a child. What on earth would you use it for?

Graves realized he had spoke too much, and tipped his hand. Killing them would be the safest bet to keep his intentions hidden.

Answer 2

It definitely appears that Graves wants Newt's obscurial. The best way to get away with the theft of something important like this is by killing all the witnesses. It can conceivably be just that simple.

If you notice, the only people present are the witnesses and the guards -- who presumably do not know too much except how to do their jobs as guards. From the guards' point of view, they see a high ranking authority doing his job efficiently. What they do not see are his not-so-good intentions. They are unwittingly used to carry out Graves' intentions of theft and murder. And it also neatly fits the enlightened response given by CyberClaw.

Theorizing a bit (because it does not appear to be part of the movie): Graves could have Confunded the guards. So they would have no perception as to what they could have seen in the proceedings. Graves could then use "normal procedures" to get rid of the witnesses.