When Tina bursts into the meeting of all the international heads of magical state, GrindelGraves takes the opportunity of their distraction to annihilate every single representative. Then he captures Tina and the case. He thinks the content of the case could be a very useful supply and has no interest in hurting the creatures for no reason (if it benefitted him more, yes, but not for no reason).

He offers Newt a deal: the No-Maj, Tina, and her sister get to live if Newt behaves and doesn't try to run off with the menagerie. Or he could just kill the four of them and take the creatures.