The Qur'an is very specific concerning punishment. Chapter 5, verse 32 describes a gruesome death for those who declare war on Islam:
(32) The only reward of those who make war upon Allah and His messenger and strive after corruption in the land will be that they will be killed or crucified, or have their hands and feet on alternate sides cut off, or will be expelled out of the land. Such will be their degradation in the world, and in the Hereafter theirs will be an awful doom; (33) Save those who repent before ye overpower them. For know that Allah is Forgiving, Merciful.The instructions for dealing with a combatant reveal a bothersome perspective about Islam, die violently or convert.