WARNING: The following contains spoilers for Attack on Titan Season 4, Episode 20, "Memories of the Future," now streaming on Funimation, Hulu and Crunchyroll.
Eren Jaeger is a much different person than he used to be in Attack on Titan, as the anime's fourth and final season has seen him change 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 near the end of Season 3. During Season 4, Episode 20, "Memories of the Future," as Grisha Jaeger struggled with his task to slaughter the Reiss family, there was a brief flashback to Eren kissing Historia's hand. This confirmed exactly what Eren saw that caused him to do everything he's done since he kissed her hand: He saw the future. Specifically, he saw this moment with Grisha and the Reiss family, including the presence of a future version of himself.
All Titan Shifters in the series are capable of seeing the memories of those who previously held their Titan. AoT had already established that when Historia, a member of the Royal Bloodline, makes physical contact with Eren, it can spark a memory from the previous holder of the Attack Titan, Eren's father, Grisha Jaeger. After the Survey Corps defeated Zeke, Reiner and Bertholdt in Shiganshina in Season 3, Queen Historia gave the survivors medals of honor in a ceremony, and each soldier placed a kiss on Historia's hand. When it was Eren's turn, the contact with Historia set off a deluge of memories from Grisha Jaeger that left Eren visibly shaken.
The episode also revealed that the Attack Titan has a special ability that allows its wielders to see the memories of future inheritors. The episode, "Memories of the Future," confirmed that the current Attack Titan user cannot willfully see the future, he can only see what future inheritors show him. Even though he's seemingly never received a memory from the future, the Attack Titan's powers still allowed Eren to see the future when he kissed Historia's hand, in an extremely roundabout way. Though he initially claimed that he saw memories of Grisha's childhood and the slaughter of the Reiss family, it's now been revealed that Eren saw much more than that.
To avoid confusion, the following references to Eren will be organized as such: Season-3-Eren, Season-4-Eren and Future Eren, where Future Eren is a version of the character who seems to exist in a time beyond what the anime has addressed at this time of writing. Judging by Grisha's words, Future Eren has been pulling the strings of the series for quite some time and whatever Season-4-Eren's plan is, it's a terrible one that's going to succeed. It's unclear if Grisha caught a glimpse of Future Eren's plan or if it was deliberately shown to him, but whatever it is it's enough to strike palpable fear in the heart of Grisha Jaeger because he saw that Eren is going to win.
When Season-3-Eren kissed Historia's hand, he saw Grisha's memory of his slaughter of the Reiss family, which Grisha did on Future Eren's orders, with Season-4-Eren present via Zeke's Founding Titan powers. Although Season-4-Eren only goaded Grisha into killing the Reiss family, Grisha later confirmed that he saw the end result of Season-4-Eren's plans that Future Eren has apparently accomplished. Put simply, Season-3-Eren saw Season-4-Eren in Grisha's memory, and through Grisha's memory, he saw Future Eren's plan. Effectively, when Season-3-Eren kissed Historia's hand, he saw his entire future.
Whatever Eren Jaeger's ultimate goal is as of Season 4, Episode 20, it's so terrible that his own father begged Zeke to stop Eren, even though Grisha had been cooperating with Future Eren's instructions up to that point. Eren's demeanor in Season 4, withdrawn and almost depressed, takes on a whole new light, as now it's clear he's been living his life already knowing the outcome of his actions. While Season-3-Eren was horrified by whatever he saw when he kissed Historia's hand, Season-4-Eren is much less conflicted about it. While the full extent of what Eren saw is still not known, that he saw so much of the future might just make him unstoppable as his fight with Zeke over control of the Founding Titan continues.