This is the severe punishment in the hereafter for the one who kills intentionally. There is a difference between killing in the fighting yard and in a normal life, we have to take into consideration the sequences of the verses, for anyone not to say that the Holy Qur’an commands the killing, Allah (swt) says:
مَن قَتَلَ نَفْسًا بِغَيْرِ نَفْسٍ أَوْ فَسَادٍ فِي الْأَرْضِ فَكَأَنَّمَا قَتَلَ النَّاسَ جَمِيعًا
(whoever kills a soul unless for a soul or for corruption [done] in the land – it is as if he had slain mankind entirely.) (Al-Maida: 32)
Killing one soul equals killing the whole mankind, indeed, this cannot be found in any worldly law.