I obviously don't know enough about the case to have an opinion on whether he should have been convicted, or whether the conviction should have been overturned. That's largely a legal issue, applied to a criminal trial where the burden of proof...
You know why mass murderers hire lawyers? Because they might find a way to get them off legally. The legal systems aren’t perfect. There are loopholes that get exposed.
Now in this specific case he changed his story like 4 times, eventually...