If he was running with incognito perk I don't think his gamer tag name would be visible. Is it possible that whilst you were shooting at him somebody else shot you from another direction? (only a thought) .
Sometimes you will get into lobbies like this where you appear to be a good second or so behind the enemy. Try another lobby.
In normal mode it doesn't take much to kill another player so in HC I would imagine it is even quicker so if you are lagging a bit that could be your problem.
If you are playing on ps4 the player numbers are so low I would suggest that that could be a cause too.
There are some known hacks and cheats out there that people are using I am just trying to offer an alternative thought as to what you are reporting.
Also if you are on ps4 just press the share button after the game and you will have your last 15 minutes of footage to go through, in my humble opinion theatre caused way more problems than it solved.
Cheers.