Is this something to be concerned about?

I have been using NIS (2010/2011) for a year now. Today I downloaded and scanned with Malwarebytes.

Malwarebytes found a file that it designated as Trojan.killAV

The log reads: Files Infected: C:\Users\----\x.exe (Trojan.KillAV)
In Quarantine it is named: Trojan.KillAV

Explorer said that the file was "Last Modified" a little over a year ago.
The file had an Icon of a red circle with a slash going through it like a No Parking sign or something like that.
Malwarebytes found nothing in the registry or memory modules or processes.

I do a Full System scan with NIS every night.

Was this file capable of doing any harm?

How bad is it that NIS never caught it?

 

send the file to virustotal.com and analyze the results.

 

What was the full (original) file path and name?

As above, upload a copy of the file to www.virustotal.com and see what the report says. Post the URL of the completed scan here, if you like.
(I think that probably complies with the TOS. It's not a HJT log.)