The belief of the Ahl al-Sunnah wal-Jamaah is that all that happens is by Allah's taala permission and it is He who creates every act. Having said this, every individual has free-will and chooses by his own volition whatever he does, thus once he has chosen it - there cannot be any objection as to why he is blameworthy for his sins. To explain further, when it is said things are predestined, it means all that happens was in Allah's taala knowledge and who had written that it must happen, before it happened. Some of these incidents have no element of man's choice, such as an earthquake, and are thus just the occurring of what Allah taala had predestined. Other things are also predestined, but man's choice is also involved (but not in an active or creative way); for example whether someone prays or not or kills some one or not. And as explained, because man's choice has participated to this extent, he is held responsible.