Blake Martin was a man assigned by the Department of Defense to investigate a series of hackings originating from NCIS but was actually later revealed to be the one responsible for the deaths of both Corporal Zack Armstrong and hacker Reed Snyder.
Born in an unknown state, Blake presumably attended college before joining the Department of Defense.
While there, he received news of illegal hackings coming out of NCIS, presumably the work of NCIS Special Agent Timothy McGee.
NCIS Season 8
Upon realizing that hacker Reed Snyder had created a videogame capable of giving whoever played it access to the Pentagon computers, Blake took matters into his own hands and became a vigilante, believing that he was acting for the greater good. He tortured and later killed Corporal Zach Armstrong whom he believed was responsible although it was later revealed that Armstrong's girlfriend, Maxine Edwards was the one who played the video game. Blake was dumb because he didn't even interrogate Zack. Blake also didn't know that Maxine was unaware that the videogame hacked into Pentagon Computers.
After disposing of Armstrong's body minus a few of the Corporal's fingers and teeth, Blake went after Snyder and later captured the hacker before torturing him to death.
With that done, Blake also began targeting Maxine but she was saved by McGee's efforts.
In an attempt to keep an eye on the investigations so that he would know when to run, Blake later arrived at NCIS to investigate McGee's hackings, having previously ignored them because he believed that they made him look bad, only for Blake to wind up in the Interrogation Room where he unknowingly confessed to killing both Snyder and Armstrong which presumably resulted in him being arrested and eventually jailed.