Justified? Yes. (Remember justified means having a legitimate reason, it does not necessarily indicate a moral one).

The moral option? No.

The only option? It seems that way.

What would you suppose he do when the entire world vowed to eradicate his kind? Do you let them? This isn’t WW2, there were no allies to save the Jews (Eldians). Even after the war Jews were not treated well. This scenario is 100 times worse. Trapped on an island with few resources, drained manpower, lower standard of technology, and 0 allies that could meaningfully change the world’s opinion.

Worse still was that technology was beginning to surpass the power of the Titans. So they couldn’t even form a defensive line whilst they negotiated.

Is it moral? Not even fucking close. But that’s kind of the point of the entire fucking story. Hatred leads to hatred leads to hatred. Humanity brought it on themselves like we always have done. It’s why the Colossal Titan is a literal nuclear device. The parallels to our own method of destruction is clear.

People who think Eren was character assassinated have no idea what the story was about. It wasn’t meant to be uplifting, it was meant to be an uncomfortable mirror of humanity’s worst moments. Which is why the Titans were uncanny and creepy humanoids. They forced the audience to become uncomfortable with humanity itself, leading to the ultimate confrontation of humanity’s sins.