When does vengeance have merit.

When The Lord sayeth it's His?

Never,

"Justice, not vengeance."
-Batman

When it is in the interest of human testing

When the outcome of the vengeance, does good for more people than letting it go.

Go go power rangers when its morphin time

When it's not Vengeance.

Never..,

When omitting to do anything would be dangerous to the public at large.

When it’s justified.