Ally is going crazy and killing everyone
Though it's still unclear who murdered the Changs, we've technically seen Ally kill one person (Roger) and shoot another (Pedro). The latter we see in real time; it's a tragic moment of no hesitation, fueled by her paranoia. The former we see via flashback. In other words, we see it in the way Ally chooses to remember it, and though it's painted as a lifesaving attempt gone wrong, there's a possibility it was more intentional than Ally would like everyone else to believe.
As we've noted before, Ally's post-election psychosis could be so severe that her perception of reality is becoming warped. In other words, she could be an unreliable narrator.
Or, even more sinister, what if Ally is just killing people who don't share her political views? One of the Changs voted for Jill Stein, and Roger is clearly a prejudiced guy at odds with Ally's worldview. Some viewers have even started asking, "What if she's the leader of the cult, and we, the viewers, are its members?" It wouldn't be the first time Murphy and Co. twisted our perception of reality with a rug-pull (see: Asylum, Roanoke).