why was the pilot to be released

The pilot from Chapter 1 is going to be released from the community because his mistake was so egregious to the community that punishment by release ensures that he will never make the mistake again.  
A key feature of Jonas's society is the Sameness. A major part of that is being emotionally neutral. The people don't really experience love, joy, happiness, etc. There also isn't fear, anger, or hate. Basically, the community is made safer by being less emotional. The pilot from Chapter 1 was a "Pilot-in-Training" and made a navigational mistake and flew over the community. Unfortunately, his flyover made many people afraid and apprehensive, and the people simply didn't know how to handle these emotions.  Because the pilot adversely messed with the Sameness, he needed to be punished. The community is told that he will be "released." Eventually, readers will learn that "released" means killed. The pilot is put to death, so that he will never upset the emotional state of the community again.