The knowledge that a network could carry out such a widespread and well-coordinated attack without being preempted, in a situation of maximum alert, will heavy on the minds of any French citizen regardless of whether victims were 118 or 119. Basically, the French security system has been revealed as completely ineffective. That is a huge problem.
How can a country possibly prevent these things while still maintaining a free society?
You can't even prevent them when not being a free society. Its not like terrorism only occurs in free societies.