"What was the reason behind the GC's executing hundreds of civilians who were not even fighters?"
Not hundreds, 57 people to be accurate in the three villages. Sevgul Uludag documents the events leading to the killings quite well, I would simply add that the people who carried them out were of a type that was identifiable in Cyprus of the 70s. Louts fooling themselves that they were patriots. They took over the three hamlets several days before the August 14 operation and robbed and raped durning their stay and finally murdered all the people before leaving. They were from a neighboring village and had no connection whatever to the five national guardmsmen from Famagusta who were handed over by the Turkish army to the TMT for execution.
From the three hamlets onwards there were no other incidents of mistreatment of TC civilians. From there to the tip of Karpasia, a distance of about 60 miles the Turkish army really excelled in summary executions and rapes. There were no army units in Karpasia, no opposition to speak of. Of the 28000 GCs stranded there, today there are only a few hundred left. The situation speaks clearly about the plan and the people who carried it out.