“West of Memphis,” the new documentary on the West Memphis Three, shows what happens when fear becomes the driving force in a criminal case rather than, you know, evidence.

It’s easy for me to see why the police and the community of West Memphis, Ark., got swept up in convicting someone, anyone, for the crime of killing three 8-year-old boys in 1993. I have a little boy. If something like this happened to him, justice would be the last thing on my mind. I would want vengeance.

If justice is a blindfolded woman holding scales, then vengeance is a blind, deaf giant who doesn’t care who she stomps on as long as someone gets hurt. That’s the feeling emanating from the crowd that waited outside the courthouse for the three defendants — Damien Echols, Jessie Misskelley Jr. and Jason Baldwin — as they were taken to jail. It’s a screaming, hateful mob whose rage comes from the grief of knowing that three little boys will never get off the school bus again and the fear that some darker force is at work.

There’s another crowd seen in the film: the celebrities who made the case a literal cause celebre and demanded a new trial almost as vocally as the first crowd demanded convictions. They, along with HBO’s first “Paradise Lost” doc, started a national movement that led to the release of Echols, Misskelley and Baldwin in 2011.

After the three prisoners walked free, a crowd waited outside to greet them — this time with cheers.

The group at the beginning isn’t so different from the group at the end. Not everyone in the scornful crowd knew the three victims; how many of them were there just because it seemed right to holler at apparent child killers? And not everyone in the cheering crowd knew the three people who walked in as teens and out as men; how many of them were there just because Henry Rollins told them an injustice had been righted?

“West of Memphis” has a sharp eye for looking at mob mentality, whether that mob is demanding blood or crying out for justice.