Will executioners of capital punishments be saved? '(...)Look, today you drive me from the surface of the earth. I must hide from you, and be a restless wanderer on earth. Why, whoever comes across me will kill me!'
'Very well, then,' Yahweh replied, 'whoever kills Cain will suffer a sevenfold vengeance.' 

Quote from Genesis 4, 14-15
God said that everybody who kill a murderer will have 7 times bigger punishment than him. Does that mean that executioners of capital punishments will be not saved ?
There is a fundamental flaw in the logic here.  

Genesis 4 clearly applies to a single person - the person who kills Cain.  Generalizing this to any executioner is not supported by the text. To wit:


  'Very well, then,' Yahweh replied, 'whoever kills Cain will suffer a sevenfold vengeance.' 


Indeed, if it did call for that, Genesis 9:6 would be unenforceable:


  From his fellow man I will require a reckoning for the life of man.
    “Whoever sheds the blood of man,
      by man shall his blood be shed,
  for God made man in his own image.


As such, generalizing adds a needless tension introduced by the hypothetical.  There is an old joke that conflates "Judas went out and hung himself," followed by "Go and do thou likewise."  Standard hermeneutical approaches say that adding to the text is not warranted.