Attack Titan holders
I am not completely sure, as I have not read the manga but from watching the anime, people who have the titan power inherit the memories of their predecessors. I think they only have the memories of their immediate predecessor, as they never showed Eren to look back past Grisha's memories. When Grisha had slaughtered the Reiss family, Eren was not born. However, he inherited all of Grisha's memories because Eren got the Attack Titan from Grisha. When Eren was having flashbacks of Grisha's memories, he had no clue about why Grisha was doing this. He had not been to the basement under their house with all the information yet. So to him, he saw his father murdering a whole family with unclear intent.