Let me ask you a very especific question. Let me ask you a very specific question.

Let me ask you a very especific question. Let me ask you a very specific question.

Have you ever acted as an vigilante before? Have you ever acted as a vigilante before?

Have you ever acted as an vigilante before? Have you ever acted as a vigilante before?

Vigilante doesn't begin with a vowel phonetically, so takes "a" instead of "an".

A naive kid walking by diwouldn't realize what was happening. A naive kid walking by wouldn't realize what was happening.

The sentence by itself is fine, but as an extension of the previous hypothetical sentence, presumably should also be hypothetical.

A naive kid who walking by didn't realize what was happening. A naive kid who walking by didn't realize what was happening.

A muscled adult man would not hesitate to act as a vigilante to prevent a criminal from robbing the elderly of their money jusright before his eyes. A muscled adult man would not hesitate to act as a vigilante to prevent a criminal from robbing the elderly of their money right before his eyes.

The elderly person can be either relieved or distraught afterbout what's comming next. The elderly person can be either relieved or distraught about what's coming next.

"The elderly" refers to all elderly people as a group, if you want to talk about a single elderly person, you need to use "elderly person", "elderly woman", etc. This sentence still isn't great, but your intended meaning isn't clear enough for me to pick the correct alternative.

The elderly can be either be relieved or distraught after what's comming happened next. The elderly can either be relieved or distraught after what happened next.