I don’t know, I think what Eren did was test to see if he could keep Reiner alive or not, and I think he let them live. His only target was Willy.
No way Eren killed Willy. Willy was torn in half. Most likely he’ll transform at the beginning of chapter 101 and start fighting Eren. I wonder what happened to Reiner though. Did he transform as well and the Armored Titan will start forming behind Eren?
We have to wait another full month before we get any answers as to what is going to happen.