Could use an explanation as to how I died...

Looks like a classic case of latency + hitbox.

You character model just like every other character models are surrounded by invisible blocks slightly outlining your character model. 
Within these boxes your model is build and rendered, Hitting inside of these boxes despite it being a hit in the model visual's will count as a hit. 
And I'm almost certain this is the case in your video. 

I made a quick sketch in microsoft paint to give an idea what a hitbox is. 
The green lines represent the hitbox and within them the model is rendered. 
It could be that a tiny part of your hitbox was still in their line of sight, besides that from your angle it might appear you're fully covered and behind the wall. 
But from their perspective a shoulder or a leg could still be sticking out and since you were on low health 1 shot would have killed you. 

Looked like the game glitched because of an "aimbot".

This game favors the shooter so if he is lagging and shoots you, the dmg will take a second to register even if you are behind cover. It feels bad but nothing we can do unless the work on their netcode.

Or he technically killed you before you rounded the corner.. but with client side hit detection, low tick rate and strange lag compensation(giving the lagger an advantage) it will look like you were shot through the wall. It's essentially the server catching up to whats already happens. Was the same with PUBG. They will have to eventually add a ping lock or it will be manipulated& exploited.. just like PUB.

You were shot in the arm that was protruding from the corner. You couldn't see him because your head was behind the corner.

I'd say limb damage is maybe too high in this game, but in this case you had just a sliver of HP left so even that doesn't matter.

P.S. Since he was standing much further from the corner (and moving in), he may have actually seen much more of you than just an arm before you even saw a part of him.

Something like this:

https://www.youtube.com/watch?v=vFf6l--ow3g&t=0m52s

figured it was something of the lag nature but was more looking to whether this is client side or server side...not a computer wiz or anything but I know I avg between 10-30 ping. I also know if issues like this become common and accepted it will be a sure fire rage inducer and most likely cause me not play. I'm very fussy about dying due to uncontrollable variables such lag spikes or glitches or bugs or opponents with mods/scripts 

Something like this is sometimes the case of desync, but in your case it really is not. You can clearly see him come out the corner just miliseconds after the killing shot, indicating that you're quite well in sync and the reason you don't see him when he shoots the killing blow is the one I listed above. You can actually see the shot pretty clearly.

These are frame by frame shots in order.. as you can see I'm dead before the bullet appears...I'm pretty sure it was NOT in sync

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Well of course you're never perfectly in sync. When your opponent shoots you, he sends that info to the server, the server agrees and sends that you've been shot to you. So there's opponent to server delay + server processing time + server to you delay. By the time your client receives info that you've been shot, you may already be out of sight of your opponent and it may appear you've been shot through walls (but in reality not - your opponent could still see you when he shot). But in your case that delay is really really small, definitely <100ms.

Also, if you can be shoot behind corners, then that means that hitreg is actually setup correctly in this game: you always shoot what you see and you can be sure your bullets will hit, that is, the shooter is always right. And keep in mind, that doesn't mean that shooter gets any advantage if he lags a lot. You will die far behind a corner, but you have to keep in mind that the shooter also initially saw you later by the same amount of time. He starts shooting later, his bullets hit later, so technically, the non-lagger has a big advantage if he choose to fight and is on even ground if he runs.

Dying behind corners may be annoying, but shooting some1 and seeing your bullets go through and hit behind the "ghost" target would be a million times worse (because your target is "officially" not where you see it and you actually need to predict target's movement and shoot air to hit the target...).

But again, in your case, the corner vision disadvantage plays a much much bigger role than delay.