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?