Eren Yaeger was a much different person than he used to be when Attack on Titan started back in 2013. By the anime's fourth and final season, he changed dramatically. This radical, almost villainous change in Eren came about after he was assailed by his father's memories when he kissed Historia's hand during the royal awarding ceremony near the end of Season 3. During Season 4, Episode 20, "Memories of the Future," as Grisha Yaeger struggled with his task to slaughter the Reiss family, there was a brief flash to Eren kissing Historia's hand. This confirmed exactly what Eren saw that caused him to do everything he's done since: he saw the future. Specifically, he saw this moment with Grisha and the Reiss family, including the presence of a future version of himself who pushed Grisha into committing mass murder.
Source：https://www.cbr.com/attack-on-titan-episode-79-spoilers-eren-kiss-historia-hand-future-vision-explained/
