They are so wrapped up in the Quran & their faith that some of them believe every verse and try to act it out.
It would be similar if Christians acted out & where serious about every verse of the Old Testament of the Bible... An eye for an eye etc.
Quran verses that mention killing infidels:
Quran (3:56) - "As to those who reject faith, I will punish them with terrible agony in this world and in the Hereafter, nor will they have anyone to help."
Quran (8:12) - "I will cast terror into the hearts of those who disbelieve. Therefore strike off their heads and strike off every fingertip of them"