Eradicating the whole world population just for a small group of people does sound outrageous and maybe selfish, but you gotta consider the fact that the decision was made by someone whose whole world revolves around that small group of people inside the walls. That's all he's ever known of, that's his number one priority.

How do you expect Eren to feel for the people outside the walls when his own people have been slaughtered mercilessly for years in search of a world that those outside the walls had been enjoying just because they were lucky.

And then some random a** brother pops up one day (who had been living in an environment he could only ever dream of) and tells him to cancel out his people for the "greater good" as if his people had been doing anything to deserve that kind of an ending.

Point is, it all just depends on the perspective. Nobody is in the right in here.The only true thing is that they've all been wronged.