But now, whether Eren Yaeger in "Attack on Titan" is a good guy or not is a heated debate among the community. While he was initially portrayed as a protagonist, the twist of his character to what can only be referred to as "evil" is actually an aspect in longer and more complex wider plot about human nature throughout history.
In fact, Eren's turn towards evil sprang from his environment: he felt that harsh world and its cruel pressure all around him had pushed him into such a role. He had become desperate--even "crazy"--for a new area of life to open up, taking beneficial changes ranging past all possible points where they might otherwise be able to succeed or reproduce before slumping back weary on the old shore-stained beach where everything thought nothing would ever change.
Shot down in defeat, his body hung upon a cross. M I D A R U, he was really the devil's successor Eren was deeply affected by generations of war. Lost lovers and the true history on both Marleyans & Eldians reflect that emotional foundation for his actions much better than anything grounds. To him, memory was something like a bottomless chasm: who knows what one might accidentally happen to fall in?
He thought that fate had arranged this or that should occur (which was why later he hated the initial necessity for such arrangements); but what actually happened, why would it have happened only to him out at all people! Erased events and truths twisted themselves in his mind more than can possibly be acknowledged--a skill at which he excelled. It was a polarizing transformation and this transformation, written in the type of style of a modern allegory, served to reflect the essence to which he gave birth with 'AoT' itself.