9.14
Fight them, and Allah will punish them by your hands, cover them with shame, help you (to victory) over them, heal the breasts of Believers,
There are also Hadiths that clearly say " So, wherever you find them, kill them, for...
One may think that the dreadful penalty mentioned here pertains to the next world. But Muhammad made sure that these people received their penalty in this world as well:
Q.16: 106
Any one who, after accepting faith in Allah, utters Unbelief,- except under compulsion, his heart remaining firm in Faith - but such as open their breast to Unbelief, on them is Wrath from Allah, and theirs will be a dreadful Penalty.
Q.3:90
But those who reject Faith after they accepted it, and then go on adding to their defiance of Faith,- never will their repentance be accepted; for they are those who have gone astray.