Even for the families.
We can't have a justice system where one of the benefits of the death penalty is preventing a future Supreme Court from ruling that their constitutional rights were violated. You posed it as a hypothetical, but as a hypothetical it's just wrong. Regardless of anyone's feelings about the death penalty in general. If someone's rights were violated our system says they didn't get due process.
Also, if a future court were to rule BK's rights were violated, it doesn't mean he walks. The plea would be thrown out or overturned and he would go to trial.
As for what went down leading up to the plea, how the families were told, handled, etc., we have one family's version. One understandably angry family. We haven't heard how the others felt about how it was handled.
JMO