Sorry, your assumption is incorrect.  If guns are not used, then knives, hammers, axes, cars, vans, trucks etc will be used.  Not having a gun has not stopped the Islamic Jihadis from killing has it?  And in fact, I would say that they especially enjoy killing with knives and swords.
Before guns, swords, spears, axes,  knives, bow and arrows were very effective means of killing and just as deadly as guns.