So, what might we do -- as in what cognitive strategies should we employ -- to prevent falling into such a trap?

Well, being aware of this issue would be an obvious thing. A lot of dealing with cognitive biases is simply being aware that they exist. And if one has any belief that is at all controversial it helps to ask every so often "why do I actually believe this?" This is especially the case as new evidence emerges or experts change their opinions.

I'm not sure we can do a very good job at avoiding this sort of trap. The level of rationality required is extremely high. Being a less vengeful person in general might help. Maybe there's some good that comes out of this. If one is ever in a similar situation just think about the Kerchers and realize that one is likely not in a circumstance to judge things well.

Only think about these things while in a dissociative state. That could be under the influence of some drugs; the other ways I know to do that are to be in a fight or a car accident.