for not wearing hijaab? or even to punish? So what this guy did was unislamic and Islam has nothing to do with the killing of the girl

For adultery there is punishment and that too if meted out by the authorities. You can call that honor punishment of lashing.

For starters How can people relate the killing of that girl to Islam. Does Islam pronounce any punishment for not wearing the hijaab? If not than the killing is not related to Islam and what that person did was unislamic. So this takes out honor killing as well. As honor killing is seen as an illegal and unjustifiable punishment for violating islamic codes.
Hope I have made myself clear.