Curate, connect, and discover
Spoiler: Cordelia killed Tatiana but Grace helped.
Okay, so looking back at the rest of the shadowhunter chronicles, someone has to kill their parent every three books (that aren’t story collections), e.g. Clary killing Valentine, Gabriel killing Benedict, Julian killing Andrew and Kieran killing the Unseelie King. Which means, that someone has to kill their parent in Chain of Thorns. And, obviously, the only way this makes sense in any is Grace/Jesse killing Tatiana.
Anyway maybe I’m going insane but I think it makes sense.