Spoilers for The Undoing Episode 4. The police may have only suspect in Elena's murder on The Undoing, but fans have several other theories about who killed her. If the show departs from what happens in the book, any one of these people could be the culprit. Let's discuss the most likely contenders.
Grace's Dad Killed Elena
There are two prevailing theories for why Grace's father, Franklin, may have killed Elena. Firstly, he could have discovered that Jonathan was cheating on Grace and killed Elena in order to frame him. Franklin seems determined to get Jonathan out of his daughter's life, and this would certainly be one way to do it. Additionally, as we saw in Episode 4, Franklin is willing to threaten people (like the school's principal) to get what he wants, and he doesn't tolerate members of his family being hurt. It's also unlikely that he would have posted Jonathan's bail if he thought Jonathan really did it, since he would be a potential risk to his daughter. But if Franklin committed the murder, he would know Jonathan was innocent and feel better letting Jonathan roam free until the evidence lands him a conviction. Plus, let's not forget that Franklin was at the fundraiser where Elena was last seen. He could have followed her and Jonathan home and killed her when Jonathan left. Reddit user rainygolf pointed out that Franklin mentioned that Elena was struck with the hammer even after she was dead, which seems like the kind of detail only the murderer would know.
Another theory, courtesy of Reddit user No_Ad, is that Elena is Franklin's daughter from an extramarital affair. We did learn in Episode 4 that Franklin cheated on his wife, and if Elena is his daughter, it's possible she became so obsessed with Grace's life because it could have been hers in another circumstance. Franklin then may have killed Elena to protect Grace or to keep Elena from revealing the truth.
Henry Killed Elena
No one would expect Jonathan and Grace's son to have committed the crime, but he may have discovered the affair and killed Elena to keep his family together. Batistasfashionsense thinks Henry could have followed his dad that night and saw him with Elena, and that's when he intervened. Perhaps Jonathan was protecting Henry in his televised interview when he suggested he knew the killer but declined to mention the name. Henry also can't seem to stop watching news coverage of the case, maybe because he's more involved than he's letting on,
Grace Killed Elena
There are also two motives for Grace to have killed Elena. One is that she discovered the affair and killed Elena in a fit of rage to frame Jonathan. Before the news of the affair was officially revealed to her, Reddit user GoddamnitHannah pointed out that Grace talked to her therapy patients about their own affair. She seemed to be speaking from experience when she accused one of wanting to be caught, as if that's what she thinks Jonathan did. This seems to also be Jonathan's theory. He suggested during his TV interview that the real killer may have acted out of jealousy. If he simply believed the killer was Elena's husband, why not say that? But if he believes that Grace did it, he'll be more inclined to keep that secret and possibly even go down for the crime in her place.
The other motive is that Grace was also romantically involved with Elena. It would explain why Elena kissed her in the elevator and also the eerily accurate portrait Elena painted of Grace. Maybe Grace became jealous when she learned that Jonathan was seeing Elena too – not because Jonathan cheated on her, but because Elena did.
Grace keeps having visions of Elena's final moments, which could just be Grace imagining things. But it could also be her playing the moment over in her mind. And she was near the scene of the crime on the night of the murder. The clues are adding up.
Jonathan Killed Elena
This is the most obvious answer, since Jonathan's DNA is all over the crime scene and he fled town after Elena's death. And as Reddit user skeematic speculated, there's a reason we saw Jonathan get into a fight at the jail. It showed how quickly he could turn from charming to violent, just like he may have done with Elena.
Whoever killed Elena, fans will get answers soon enough: there are only two episodes left in the series.