Primal Fear ending explained

What happens

After Aaron is acquitted by reason of insanity, Vail visits him one last time. A slip shows that the violent second self, Roy, never existed. Aaron acted throughout and killed the archbishop himself. Vail walks out stunned.

The last image

Vail leaves the cell in shock, understanding that his own skill helped set a murderer free.

What it says

Vail sold an image and was beaten by a better performance. The ending separates a legal verdict from moral truth: the court was satisfied, his conscience is not. He was fooled by what he wanted to see.