You're his enemy. You sided with the ministry and opposed Grindelwald's wishes to enslave no-maj/muggle. He thought that no-maj were just a bunch of eye-searing pests and want to banished them. You are on your mission to disrupt the meeting of Grindelwald and those who wish to follow his will. He is very good at manipulating people and you need to be very careful when confronting him.

"Come and join me. Together we build our own world." he said gracefully.