Ive been killed several times with a single bullet when my energy bar is near 0 and this doesn't happen because i didn't activate armor because even without armor and 100% energy i don't get killed as quickly.

This is one of the reasons Assault is unbalanced, any normal soldier can kill a guy that looses health when he runs but it takes a really good NS player that knows how to use his energy to take down one of those "normal" soldiers ... that can regenerate health

Energy is not health, its a seperate resource. The only time energy influences your resilience is when you are in Armor mode in which case 35% of incoming damage is diverted to suit energy.

Energy its like an armor, but isn't your total health.

Try it out Get killed with energy and without energy

sometimes you even have the time to turn around and shoot the attacker in the head if you use armor mode and the attacker not.

Questioning himself: What the ****, it's not fair, and it's a bug.

Even without the armor mode on, you may be lucky.
Two reasons, you are more accurate and the other is that the opponent is less.
But it's situation sensitive.

cloaked players are likely to have less health than normal, as you tend to die very quickly when busted, I think

I think cloaked players have 0% armor, just own health
visible players have about 100% armor, + own health
and armored players 150? or 135% like said + own health