One of these memories was when Grisha stole the Founding Titan from the Reiss family.
He is part of the Survey Corps and is willing to protect his village from the Titans, even if it means destroying the rest of humanity outside the walls.
Despite Eren's continued pleas for her to join them and prove her innocence, Annie transforms and he, Mikasa, and Armin are forced into the underground tunnel to escape from her Titan.