
To create a world that paradis people can live happily without the fear of being massacred and invaded by the outside world. It was mentioned in the story that the people beyond the ocean sees them as devils and must be eliminated. In order for that not to happen, eren took it in his own hand the safety of paradis. Even if it cost him to be the devil himself.

Great summary. In addition, I think he's trying to become the "evil" that the other titans + Survey Corp members join hands to defeat. In the latest chapters, it's seen that people all over the world are vowing to live as virtuously as possible if they survive Ragnarok (Isayama has stated that AOT is based on Norse mythology). By Eren becoming the sole antagonist, his friends -and by extension, people of Paradis and global Children of Yumir- can be seen as the saviors of the world.
Okay can someone tell just what is happening with eren??
I watched all aot and seen some spoilers but why is he doing THIS!?