Newt is captured by an escaped Grindelwald and his minions and his knowledge of magical creatures is useful and he's a handy hostage to have being the brother of Theseus Scamander and a favourite of Albus.

Newt has been his captive for a while when Grindelwald walks in on him being raped by one his followers

Grindelwald pulls the guy off Newt and kills him on the spot

"There are lines Mr Scamander that even I will not cross"

Please nothing between Newt and Grindelwald.