so I don't understand it fully but what I could get is that Levi killed Eren back when he was a captain and that Eren loved him but couldn't get the chance to tell him. Eren reincarnated and was looking for Levi everywhere but couldn't find him so he created his own imaginary world where he meets Levi by accident and is in love with him. hence the last scene where the imaginary Levi tells him that he doesn't regret killing him nor the stupid world but Eren didn't want to hear that his happiness is just in his mind so he killed the imaginary Levi to shut him up.
so I don't understand it fully but what I could get is that Levi killed Eren back when he was a captain and that Eren loved him but couldn't get the chance to tell him.
Eren reincarnated and was looking for Levi everywhere but couldn't find him so he created his own imaginary world where he meets Levi by accident and is in love with him. hence the last scene where the imaginary Levi tells him that he doesn't regret killing him nor the stupid world but Eren didn't want to hear that his happiness is just in his mind so he killed the imaginary Levi to shut him up.