How sad... they haven't learned a thing from the mistakes of their past. It's a tragedy that King Reiss took their memories away from them. Because of that, they make the same mistakes over and over again

My interpretation is that he's talking about the war which predated the exodus of humanity into the walls. This gives us a clue about the nature of the conflict, if so - it suggests that King Reiss' faction were at war with Zeke's faction, and the Reiss' army were on the verge of losing. Then there was some hopeless suicide charge, but somehow King Reiss managed to win and bring humanity to safety within the walls.

Could it be that Zeke's father was King Reiss' adversary? In that case, Zeke won't be the first person to inhabit the Beast Titan form, and it was instead passed down through the generations. The same goes, perhaps, for Reiner (see: "Armor - Braun" serum) and Berthold. Maybe Zeke's father balked at the prospect of wiping out humanity, and left an opening for King Reiss to steal the co-ordinate and bring humanity to the walls?

Okay, hold on. If Zeke's homeland is a place where one can live comfortably enough to play games, drink coffee, whatever... then why hold a grudge against this society that seems worse off?

If the titan shifter faction is one side in the war which predated the mind wipe and the exodus to the walls, then it would seem that the walls are constructed out of the shifter faction themselves. Perhaps Zeke et al consider the state of the titans which make up the walls a de facto genocide of their own people, since the walls can't exist without thousands and thousands of them being held in stasis indefinitely.