The full, terrible effects of the Rumbling on the rest of the world were demonstrated in the most recent Attack on Titan anime episode. It's difficult to imagine the tenacious but kind Eren Yeager from the show's early seasons as the seemingly detached monster who is now committing one of the worst crimes ever depicted in an anime.
However, Eren's character had already been set up for growth from the start. His first encounter with the Female Titan outside the Walls is the best illustration of how each of his tragic experiences contributed in some way to his final decision to pursue the Rumbling. In the Forest of the Giant Trees, Eren witnessed the brutal deaths of the Levi Squad. How did this event foreshadow Eren's justification for the Rumbling?
Eren's Tragic Choice When the Female Titan Appear
Eren's first expedition outside the Walls sees him closely guarded by Levi and his special squad members, all hand-picked for their abilities and quick decision-making. While under pursuit by the mysterious Female Titan, Eren is stopped by Levi from transforming into his Titan form. He tells Eren that there are no inherently right answers in the heat of the moment, and ultimately it's up to him to make a choice he can live to regret the least: either he trusts in the experience of his superiors and in his comrades, or he can rely on his own strength. Eren decides to believe in the Survey Corps, and is rewarded in the short-term by witnessing the Female Titan's capture through Erwin's masterful trap.
However, after Annie escapes and once again goes after Eren -- this time without Levi around -- Eren is again given the same choice. Petra and Eld ask him if he has that little faith in their abilities, and Eren once again leaves the Female Titan to them. Unfortunately this time, his inaction leads to devastating consequences. Eren is forced to watch as Petra, Eld and Oulo are all brutally murdered, and he is captured regardless. Eren's regret and suffering is clear on his face, and the viewer too realizes that if he had transformed and fought alongside the Levi Squad, they likely would've had better odds of apprehending the Female Titan with fewer if not no causalities.
How Eren's Repeated Reliance on Others has Failed Him in Attack on Titan
Eren's encounter with the Female Titan is one of the first times he is faced with a difficult decision and makes the wrong choice. He chose the more traditional "anime" option of putting his trust in his friends, only to be confronted with the harsh reality that he should've followed his instincts and taken an active role in the conflict. This lesson has a huge impact on Eren's psyche, and it's visible in his decision-making throughout the rest of the series, despite the fact that it's rarely mentioned. While not adapted for the anime, Chapter 130 of the Attack on Titan manga depicts a scattering of important memory shards for Eren, one of which is of the Levi Squad.
Eren's key takeaway from his failure in the Forest of the Giant Trees is that if he leaves conflicts up to others and puts faith in his comrades, he is rewarded with suffering and regret. This comes up again in Season 2, when Eren places his trust in Hannes' ability to protect him and Mikasa from the Smiling Titan, only to watch one of his oldest mentor figures get crushed. In Season 3, Eren trusts in Armin's plan to defeat the Colossal Titan, and then finds his friend's charred, barely-breathing body.
Every time Eren puts fate in the hands of someone else and chooses to believe in his friends and commanders, he is punished with more death and tragedy. By the start of Season 4, Eren has learned an important lesson: if he wants to protect those he cares about, he must be the one to act.
Eren's Final Decision: The Rumbling
At the start of the "War for Paradis" arc, Eren goes off on his own to Marley to make the first move, a decision met with confusion and disapproval from his friends. Yet Eren's reasoning for making this move is displayed when he speaks to Hange while in jail. He becomes angry and frustrated with her inaction, and violently grabs her and asks what she can do. This outburst shows Eren's inner conflicts: he doesn't want to murder innocent Marleyans, and he certainly doesn't want to go through with the Rumbling and murder the millions who live outside the Walls. Yet no one else has a concrete plan to save Paradis, and Eren has learned firsthand what his inaction leads to.
Rumbling in Attack on Titan is motivated primarily by Eren's desire for freedom, but also by his desire for his friends to live long and happy lives. These two desires are at the heart of his character, allowing him to proceed with his plan for the Rumbling despite the obvious weight of his horrifying decision. Since his first meeting with the Female Titan, whenever Eren has left fate in the hands of others, regardless of their greater experience or skill sets, he has always suffered as a result.
Eren could sit in Paradis with his friends, attempting to find a more diplomatic solution to the Eldian problem as the outside world prepares to wipe them all out, but he knows time is running out for him and Paradis. For Eren, there is no other choice but to keep moving forward and destroying the rest of the world before his inaction leads to the deaths of those he cares about and the destruction of the place he calls home once more.