I have a question about medkits. Sometimes when I get shot my character died instantly and sometimes I get a chance to revive, in both instances there is medkit equiped in my inventory… is that a thing or bug? Can someone who knows better mechanic explain that 
thanks
A headshot is always instakill, no matter if you carry a medkit.
Anywhere else, I think there’s a certain %
There is a chance that you will be downed when you recive critical damage (but like @114491451 said - headshot kills instantly). That chance can be increased by one of the perks (that is absurd and should not be a thing)
As has already been said,
Fatal damage, like that from tank guns/ Mgs, artillery or high power rifles will instantly kill you, Medkit or not.
Standard rifles, smgs and pistol increase the chance of being downed, but can still kill.
Every weapon has a certain dmg output and you have a certain hp. If the dmg is way over the hp you have you insta die. Walking around already taken prior dmg lowers your hp and increases the chance to get insta killed.