Back in April a man named John Thompson wrote an op-ed in the New York Times about how he had ended up on and then been exonerated from death row. John Thompson was alive because evidence the prosecutors had hidden for 15 years was found, in a last-ditch attempt by his legal team, just a few weeks before his scheduled execution.

If you support the death penalty, I hope the preceding paragraph encourages you to question it. But I’m more interested here in what exactly the prosecutors were thinking.

We know what the prosecutors did. They had blood evidence from the scene of an armed robbery which pointed clearly at someone other than John Thompson being the robber. But they deliberately didn’t tell anyone about this evidence. In particular, they (illegally) didn’t tell the defense. So John Thompson was convicted of armed robbery. It was an easy step after that to convince a jury that he was guilty of a separate capital murder, which he also hadn’t done. And, oh yes, they had witness evidence, which they deliberately did not tell the defense, that he wasn’t the murderer.

What were they thinking? We have a pretty good idea. They had their guy. Their job was to convict this guy. They probably convinced themselves that he was indeed, despite the evidence, guilty. Maybe they convinced themselves the evidence wasn’t so important after all, so it didn’t matter if the defense didn’t know about it. Maybe, on the other hand, they convinced themselves that it was so important it had to be suppressed because it was even more important, way more important, to convict their guy. Because they’d convinced themselves he was guilty. And we’re back where we started.

Why do we have such a good idea of what they were thinking? Because everyone does stuff like this all the time. Let’s take it step by step.

We start off with the convenient thing to do (prosecute the guy the police arrested). Then comes self-interest (we want to win this case!). Self-interest is, almost inevitably, followed by delusion. And when we’re deluded we not only tend to insist we are right, we tend to get really self-righteous about it. And once we’re sure we’re right in a most self-righteous manner, we start cutting corners, taking shortcuts, shoving the dust under the rug along with the chair, the couch and the refrigerator. We cheat. We lie. We do terrible things. But hey, that’s okay, because it’s in a good cause, a cause so good that it excuses whatever we’re doing.

If you haven’t noticed this in your own life, you haven’t been paying attention. For most of us the stakes are fairly small — not many people get to prosecute capital cases. So on those rare occasions we admit to ourselves what we’re doing, we can pretend that it doesn’t matter.

But it does. It always matters. No thing is too small to matter. Be careful, always and everywhere.