I would go further and say that this event of the truth of "release" and seeing his father kill one of the twins sparks of a kind of epiphany for Jonas - he suddenly realises and pieces together lots of truths about his community that he had suspected or noticed but had only felt uncertain or frustrated by. These feelings which Jonas never really gives speech to or acts on are triggered by the "final straw" of the release of the twin and the role of his father in the twins death. This event is what gives Jonas the courage he needs to take the step the novel has been leading up to - his escape with Gabriel from the community.
I agree completely with posts 2 and 3. I think that not only does this open Jonas' eyes to the truth of what is happening in the community and the lies that it is built upon, but I think this really shatters illusions he had about everyone in the community and their understanding of the world. He realizes that his father, who is a nurturer, who loves children, is gentle and caring, does this terrible thing, and doesn't even bat an eye. His father is so indoctrinated with the community's rules and institutions that the father doesn't even realize this is murder. To the father, and most people in the community, it is not seen as murder, but just something that must be done. I think this inhumanity was what Jonas saw and realized that he could no longer live in the community.
Most people say that Jonas watching his father release the little baby is the climax of the book. This is because that is the last straw that makes him decide that he cannot live in the community anymore.
When he sees his father release the child, he comes to realize that his whole community is built on lies. He starts to wonder how many people get instructions saying they may lie. He also gets disillusioned about a society that can care so little for human life that it would do that.
His feelings were sad for the twin boy after watching the vedio, and mad and disgusted at his father for killing the blameless newborn.
Before the video about father releasing a baby, he didn't know what it is and he guessed it was blameless. After the video he worried about Gabe and took him to escape from the community.
Prior to witnessing the actions of the release I believe that Jonas did identify his father as a nurturer. He had seen him care and advocate for Gabriel to be able to have a later release. He had also recognized that maybe all his families’ feelings were not like his, but he was unprepared for the real truth of his father's ability to release the infant.
Jonas observed his father's interactions with the two infants. He had expected that his father was automatically going to give the infant its shots. When he realized that his father was capable of murdering the infant without any second thoughts or compassion or emotions about the incident, Jonas realized that he was not part of the members in his family any longer. He could not be a part of the community.
When Jonas looked around him he saw people capable of doing horrible things with no idea of their actions or no emotions. He knew he had to leave.