Was hanging Mary Surrant with the Lincoln conspirator's justice? What crime was she supposed to have committed?

Well, the conspirators met at her boarding house, as I recall, so the view was that she was complicit.

Many people thought she'd be reprieved, but it's possible that President Johnson was kept from receiving the clemency request.

Was it justice? Well, opinions differ on that. Some people think she knew what was going on; others don't think so. Her son was a Confederate spy, and she may have known about that, too.

Aiding and abetting treason and murder.

How about conspiracy to murder the president?? The evidence was mostly innuendo, and circumstantial. But given that we had just been thru a brutal civil war; and hatreds, tensions were running high against all Southerners not all that surprising.

The verdict was a foregone conclusion. That's why Stanton used a military tribunal. They were all civilians. You don't get to try them in a military court. That's a tad unconstitutional.