Some people claim that those Mongolian flowers are poisonous and that Ein ate one when Zwei died, but I've researched and there is no reliable source that tells us that those flowers are really poisonous. Some people claim that actually, they weren't together, Zwei was somewhere else, and others claim that Ein killed Zwei.
Did both Ein and Zwei die? What did really happen there?
The anime ending is different from the visual novel.
Ein and Zwei were definitely together in that last scene because Ein was talking to him.
The poisonous flower idea is a fan theory without official source. There are no official explanations of what happened in the last scene. It does make sense, though, to assume Ein committed suicide after Zwei got shot, and the flowers are the best and easiest tools for her suicide.
Since there's no official explanation, you can go with whatever idea you want.
It actually makes the watcher confused.
In the anime, Reiji was unexpectedly shot by an assassin (a man on a cart).
They were in Mongolia's countryside when it happened (Reiji was also not too far from Eren).
After it, we see a smiling Eren, a flower, then the bullet that killed (?) Reiji on the ground and we see this flower again but one of its petals has disappeared.
There are several fan theories about it. Still, none have been confirmed.
I think the producers just wanted a striking end and that's it. This is a shocking end.
