THREAD

To reiterate my thoughts, Eren hasn't "switched to the dark side". Eren's always had a fundamental darkness to his character, it's more accurate to say he's succumbed to it. He is naturally prone to violence & has an intense bloodlust, and is a fanatic, here are examples:
1. Finding out he wasn't free, he developed an angry and unhealthy obsession towards wanting to achieve that freedom. It woke his anti-social traits (anti-social doesn't mean introverted)
2. Consistently getting into fights with people who disagree with him, such as Armin's bullies or Jean in the 104th cadet training corps
3. Killing grown men as a 9 year old. Im sorry no child is supposed to not have any remorse or not experience negative mental aftereffects. He felt fully justified and doesn't regret anything. Not saying he's wrong to help Mikasa, but what he did is unnatural to a normal child
3.5: He debases them calling them monsters, when he happens to behave like a monster himself in this moment
4. Intense feelings of revenge after Shiganshina district was decimated and his mom eaten. The intensity is frightening, pure and raw hatred, you can just feel it through his expression
5. The extreme determination that even Keith sensed to be frightening and that he couldn't put a stop to. Eren has unmatched willpower. It's not the willpower that's dark, it's the fact that it's driven by something as negative as revenge (at the time, also his dream for freedom)
6. This confession to Levi with that sadistic face
7. Pure and alarming bloodlust upon first turning into a titan
8. Sense of purpose reignited by his obsession with his dream. It's not wrong to have such a dream & to work for it, but him & Armin share the same dream & Armin is going ab it in a normal & healthy way. Besides, as mentioned, his sense of purpose is frightening
9. An example of concerning bloodlust against the female titan, being so blinded with rage he again loses a battle (like he did in Trost jumping straight into it on pure rage)
10. EXTRA!! not included in the Manga!! He goes berserk on Annie, making declarations that have since come to pass
11. Promising Reiner and Bertholdt a world of pain. Again it's normal that he feels awful for being betrayed but by any objective standard what he's saying is not a sign of normalcy or upstanding mental health
12. "If we kill our enemies will we be free?", the first real hint of his darkness popping out, or rather, foreshadowing for what he's about to do
13. Post time-skip now. Starts to display anti-social traits. One of them is manipulation. He manipulates Zeke. Uses Zeke's issues against him to gain his trust then betrays him, leaving a painful aftertaste in Zeke's mouth
14. Manipulates Falco. Gains his trust to do favors for him, later betrays him in the basement with Reiner.
15. Manipulates Grisha and Kruger. Grisha to get the Founding titan's power and Kruger he sends him some memories to push Grisha into his mission. This leaves Grisha defeated and broken
16. Destroying Liberio. Eren shows cold-heartedness and ruthlessness. He's killed ppl and even children and took the Warhammer's power quite brutally. He also dragged his friends into it since he put them in a position where they'd essentially have to go help him
17. Lying to his friends. It's not the lying in itself but more so an expansion of his current callousness in his actions. He hides from them his intentions and makes Armin doubt himself and tells Mikasa he hates her
18. Planned betrayal with Floch and Historia where he openly admits he wants to eradicate humanity. He, again, debases humans and calls them animals to Floch here. And he thinks he's entitled enough to go out and erase hate from the world.
19. He even displays scary determination when he goes and mutilates himself for his goal. Shows how extreme he is in his actions
20. Activates the Rumbling. Need I say more? He doesn't even look human anymore
21. Finally, again pushes his friends into a corner by pushing them to have to choose him or the world. It's not a kind act and he knows it. Reinforces the notion he wants to be stopped, but won't go down without a fight. Also looks devoid of humanity
END OF THREAD

Disclaimer: that's not to say Eren's immoral from start to finish. He just went from one extreme to another displaying his inherent emotional instability and maladaptive personality traits. His actions are uncontested evidence of his nature, he admits so himself:
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